Wolfram Language (Mathematica), 37 bytes

Prime[Fibonacci[(c=⌈Pi⌉)!-c]+c]+c

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67961

brainfuck, 27 bytes

-[----->+<]>--.++++..++++..

Output 15599. I am fortunate that my user ID is made of increasing digits. The initial loop sets a cell to ASCII 51 = 255/5 which is the character 3 So it would be even better if my user ID started with 3, but you can't have everything.

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MATLAB / Octave, 16 14 bytes

'Ｚ'*['⦺']'

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I was inspired by another answer which used matlab's very weak typing to subtract space from a string to get each digit of their id. I figured I could do something even worse: linear algebra with unicode characters!

matlab thinks of strings as an array of utf-8 bytes, and lets you abuse this without any consequences. read literally, we're taking the matrix product of 'Ｚ' with the transpose of '⦺' (basically a fancy way of doing a dot product).

utf-8 needs to represent all of unicode while still being backwards compatible with ascii. if you look at the binary, all ascii looks like 0xxxxxxx, where xxxxxxx is the unicode index of that character in binary, so it keeps all of that the same.

to reach the rest of unicode, utf-8 uses a cute length-thing: 110xxxxx means 'this character uses 2 bytes', and 1110xxxx means 'this character uses 3 bytes' and so on. all the 'extention' bytes that come after the first byte are marked with 10xxxxxx to show that they are part of the previous character. (so unicode chars with an index under 11 bits look like 110xxxxx 10xxxxxx, unicode chars with an index under 16 bits look like 1110xxxx 10xxxxxx 10xxxxxx, and unicode
chars with an index under 21 bits look like 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx)

first, some analysis: with 3 bytes, the ascii range only gets up to 47628, with 4 bytes reaching 63504. neither of these are close to my id (119818). we actually would need at least 8 bytes on both sides if we were limited to ascii chars

however, non-ascii utf-8 bytes must necessarily have the top bit set, meaning they are much bigger. instead of each number sitting between 32 and 126, continuation bytes are between 128 and 191, the initial byte of 2-byte chars is between 192 and 223, and the initial byte of 3-byte chars is between 224 and 239. this gives us a range of roughly 53k to 86k for 2-byte chars (which includes 119818/2=59909), and 83k to 130k for 3-byte chars (which includes 119818).

I'm more familiar with julia than matlab, so I searched for 3-byte possibilities over there:

julia> collect(reinterpret(reshape, Int64, findall(iszero, 
[[224+a 128+b 128+c]*[224+d;128+e;128+f])[1]-119818 for a=1:15,b=1:63,c=1:63,d=1:15,e=1:63,f=1:63])))
6×15056 Matrix{Int64}:
 14  13  15  14  13  14  13  15  14  13  15  …  13  13  13  13  13  13  13  13  13  13
 63  63  63  63  63  62  62  59  63  63  63     22  21  20  19  18  17  16  15  14  13
 63  63  62  63  63  63  63  62  63  63  58     54  55  56  57  58  59  60  61  62  63
 13  14  15  13  14  13  14  15  13  14  15     14  14  14  14  14  14  14  14  14  14
 63  63  59  62  62  63  63  63  61  61  61     63  63  63  63  63  63  63  63  63  63
 13  13  14  14  14  14  14  14  15  15  15  …  63  63  63  63  63  63  63  63  63  63
julia> filter(x->all(isprint.([x...])), 
[Char(ans[1,i]<<12+ans[2,i]<<6+ans[3,i])*Char(ans[4,i]<<12+ans[5,i]<<6+ans[6,i]) for i=1:15056])
8436-element Vector{String}:
"ᢾ콾"
...

after filtering down to only pairs of printable characters, we still have 8436 options. 446 pairs have at least one lowercase char, 272 have uppercase, 272 have a punctuation, and there's no digits. 24 pairs have no letters, 1100 have one letter. unicode range is 0x18be to 0xfffd, with more towards the end of the range; so it's mostly cjk and hangul. ended up picking
[224+15 128+60 128+58]*[224+2;128+38;128+58] = 119818
or
[FULLWIDTH LATIN CAPITAL LETTER Z]*[CIRCLE DIVIDED BY HORIZONTAL BAR AND TOP HALF DIVIDED BY VERTICAL BAR]'

Old answer:

'Ňل'*['ʕł']'

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since my userid is even, I originally aimed for userid/2, looking for 2-byte unicode characters that work. since id/2 is closer to the boundary here, there are only 13 pretty pairs of characters which multiply to half of my userid, so you can see them all: (first four fit in ascii so are invalid, next four have unprintables, then there's some hebrew punctuation character that doesn't show up on stackexchange)

18×2 Matrix{Vector}:
 [69, 1545]   ['E', '؉']
 [73, 1541]   ['I', '\u605']
 [80, 344]    ['P', 'Ř']
 [88, 336]    ['X', 'Ő']
 [131, 1733]  ['\u83', 'ۅ']
 [133, 1731]  ['\u85', 'ۃ']
 [137, 851]   ['\u89', '͓']
 [147, 841]   ['\u93', '͉']
 [322, 1604]  ['ł', 'ل']
 [323, 1478]  ['Ń', '׆']
 [324, 1602]  ['ń', 'ق']
 [326, 1475]  ['ņ', '׃']
 [327, 661]   ['Ň', 'ʕ']
 [341, 647]   ['ŕ', 'ʇ']
 [449, 1102]  ['ǁ', 'ю']
 [462, 1089]  ['ǎ', 'с']
 [707, 1031]  ['˃', 'Ї']
 [711, 1027]  ['ˇ', 'Ѓ']

since a dot product is a linear operation, you can stick two pairs next to each other and they add together, so this solution is
[192+5 128+7]*[192+10;128+21] + [192+25 128+4]*[192+5;128+2]=59909+59909=119818

Wolfram Language (Mathematica), 86 66 bytes

Saved 20 bytes thanks to an idea of DLosc!

Last@First@EntityValue[Interpreter["City"]["Tyler MN"],"ZIPCodes"]

Not super short, but fun: look up a city's ZIP code 😄

Previous code:

Last@First@EntityValue[Entity["City",{"Tyler","Minnesota","UnitedStates"}],"ZIPCodes"]

JavaScript (ES6), 18 bytes

Suggested by @l4m2

Computes +0xe4c3 = 58563.

_=>+atob`MHhlNGMz`

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JavaScript (ES6), 22 bytes

This computes \$58563=242^2-1\$, where \$242\$ can be converted to MjQy in base-64.

_=>(q=atob`MjQy`)*q+~_

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This is 1 byte longer than ...

JavaScript (ES6), 21 bytes

... the boring answer.

_=>"".charCodeAt()

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Python, 10 bytes

b"t".hex

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Creates and returns 101374 as a hex dump. Contains two unprintables.

Python, 23 bytes

-7 @xnor by printing directly avoiding string interpolation

print(*b'\n\rJ',sep='')

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Python, 30 bytes

print('%d%d%d'%((*b'\n\rJ',)))

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Writes 101374 by concatenating ASCII codes of \n,\r,J.

Or:

Python, 18 bytes (@xnor)

print(ord("𘯾"))

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Vyxal, 5 bytes

»ṡṫ»d

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Outputs 78850 by doubling the compressed number 39425

Ruby, 10 bytes

쯡 has character value 52194.

p'쯡'.ord

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Ruby 1.8, 21 bytes

print'fq'.to_i(?!),?^

My original answer, here just for posterity's sake cuz it was more creative.

fq is 521 in base 33. Since ! has a byte value of 33, we can access it with ?!. Same with ?^ for 94, combining into 52194. No longer works in Ruby 1.9+ because in that version ?x symbols give one-character strings instead of the character's byte value.

Google Sheets, 13 bytes

=CODE("𝞏")

46 bytes without CODE.

=LET(a,--TRUE,b,a+a,c,a+b*b+b,a&b&a-a&c&a&b+c)

31 bytes using ARABIC ^{credit to Squishalot}

=ARABIC("CXX")&ARABIC("DCCXIX")

05AB1E, 3 bytes

ŽÍ¿

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Simply uses the compressed 52210, which is output implicitly. 🤷

See this 05AB1E tip of mine (section How to compress large integers?) to understand how the compression works.

APL+WIN, 16 bytes

⌽(⍕⎕av⍳']_')~' '

Obtains index positions of ]_ characters in the atomic vector, converts to characters, removes the space and reverses to give 6949

Nekomata, 5 bytes

"Hᶜ"∏

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My user ID is 9288.

"Hᶜ"∏
"Hᶜ"    The string "Hᶜ" ([72, 129] in Nekomata's custom encoding)
    ∏   Product (72 * 129 = 9288)

Red, 52 bytes

f: func[s][load debase s]prin rejoin[f"NzU="f"Njgx"]

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The straightforward base64 encoding of "75681" contains digits - that's why I ended up joining the encodings of "75" and "681".

JavaScript (Node.js), 32 bytes

_=>[a=(b=-~-~!_)-~b]+~-a+b+~-b+b

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Not assuming any char encoding

JavaScript (Node.js), 16 bytes

_=>atob`NzYzMjM`

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Use base64. unicode would go longer so not here

PowerShell, 37 bytes

-join("`a",'S','_'|%{[byte][char]$_})

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Not strictly a Unicode solution: takes the string representation of some ASCII codes; "`a" is the 'special' character in PS (Alert/Bell, 7)

Three more solutions more in the spirit of Steve's comment "I'd encourage people to also submit an alternative version that doesn't rely on simply decoding a unicode character"

PowerShell, 76 bytes

$o++
$f=$o+(++$o)*$o--
"$([math]::Cos(($o++-shl$f+$f+$f)-$f))"|% S*g(++$o)$f

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The cosine of 32763 happens to be -0,783955601801536
This constructs a one, a five, shifts the 1 left by 15 bits, subtracts 5, takes the cosine, turns it into a string, and then invokes Substring(3, 5).

PowerShell, 80 bytes

$t++;$t++
$i=$t*$t*$t
-join$((--$i)
(++$i)
$i/=$t
(--$i)
($i*=$i)
$i++
($i/=$t))

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Creates a 2 and then uses a variable to create the digits; the expressions in brackets will also be output, and then -joined to a single string.

PowerShell, 119 bytes

$p="$([math]::PI)"
$t++;$t++
$i=$p.Length
-join$($p[($i-=$t)]
$p[($i-=$t)]
$p[($i-=$t)]
$i-=$t
$p[($i-=$t)]
$p[(--$i)])

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This iterates over the first few decimal places of Pi, which contains my user id backwards.

3.14159265358979
     ^^   ^ ^ ^ 

Tcl, 18 bytes, 16 chars

puts [scan 犍 %c]

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Jelly, 3 bytes

⁽Be

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R, 29 bytes

`+`=strtoi
"ajde"+("l"+pi^pi)

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My user ID is 101276. Converted to base-21 this will be AJDE, which contains no digits 0-9. Now to get 21 I am using the same base-conversion function strtoi with the arguments 'l' and \$π^π\$ which yields 36 (the decimal part is ignored).

R, 62 bytes

b=pi^pi
`^`=strtoi
format(as.Date('d'^b*'d'^b*'f'^b),'%d%m%y')

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The second - quite a long solution - outputs the UID as a date without delimiters (December 10th, 1976). The function as.Date receives 2535 as it's argument. Luckily, 2535 is a product of 13 * 13 * 15 and these numbers are obtained by base-36 conversion as shown above.

Zsh, 18 bytes

[
<<<$[$?$#?#ABII]

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[ fails with exit code 2, so $? = 2. The string 2 has length 1, so $#? = 1. Then $?$#?#ABII interpets ABII from base 21, which gives 97857.

A translation of GammaFunction's boring solution also works:

Zsh, 12 bytes

<<<$[##𗹁]

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Nim, 44 bytes

import unicode
echo int "𝥒".runeat len []

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Nim (without unicode), 68 63 bytes

import math
var a,b,c=on.ord
a+=a;b+=a;c=a^b+a
echo a*b^c+b*a^c

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Explanation:

OEIS A094125

3*2^10 + 2*3^10 = 121170

Brainfuck: 35

id: 128398

-[>+>+<<-----]>--.+.>+++++.<+.>+.-.

Bespoke, 52 bytes

its Josiah R W:developer,and this languages creato-r

That's me!

Vyxal 3, 4 bytes

Ꮠ≜ƛd

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Same as my v2 answer, except 39425 can be compressed as Ꮠ≜ƛ for 3 bytes instead of 4

Retina 0.8.2, 12 bytes

bhgac
T`l`d

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ACCUMULATOR, 18 bytes

Expects no input.

AOOMOAAAAAAOAAORAO

MATLAB / Octave, 22 bytes

disp(['SVSYX'-' ' ''])

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Explanation

disp(['SVSYX'-' ' ''])

      'SVSYX'           % Row vector of chars
             -' '       % Subtract this char (space). Converts chars to codepoints
     [            '']   % Concatenate with empty string. Converts codepoints to chars
disp(                )  % Display

Perl 5, 16 bytes

say"@@@@@"^wrwvw

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TypeScript (TS type system), 101 88

type L<X>=X["length"]
type _=`${L<[,,,,,,,,,]>}${L<[,]>}${L<[,,,,]>}${L<[]>}${L<[,,,]>}`

Has errors, but type _ is "91403".

Wolfram Language (Mathematica), 24 bytes (81203)

⌈°!E!/°^E/(Pi-°)⌉

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Yes, ° is a number.

Uiua 0.17.0-dev.1, 10 bytes ^SBCS

/-"%÷η÷τ𝄐"

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I decided I wanted to work within Uiua's SBCS, so this computes 117598 using only the values of Uiua+ASCII characters. /- is an alternating sum of the string, which works since Uiua treats characters as the null character plus their Unicode values.