Google Sheets, 24 22 bytes
=bul(A1,"=bul(A1,"")")
Put the character in cell A1 and the formula in B1. Gets the 1-indexed first occurrence of the character in the code, or an error value if it's not present.
Google Sheets supports international function names, and bul() is find() in Turkish. It is a valid function name in any locale, and will get automatically translated to the spreadsheet locale if it's not Turkish, and will continue to work after translation. To avoid translation, set File > Settings > Locale > Türkiye.
In an English locale, you can get the equivalent like this (24 bytes):
=find(A1,"=find(A1,"")")
A variation of =find(A1,formulatext(B1)) would work too, and there are shorter names for those functions in other languages, but that would violate self-referential rules.