How can I write the following figure in MuseScore?
This is the best I could do so far: (I made ties through the low E notes, but still MuseScore doesn't turn it into a whole note.
Fairly simply. You see at the right of the toolbar at the top numbers 1 to 4? Those are voice numbers. Voice 1 is represented as blue when you enter it, and it works with up-and-down stems if you enter only one voice. However, if you choose Voice 2, the notes you enter will show as green and use down stems for a lower voice, and any notes entered in voice 1 will all use up stems. That's to say that you will have two separate voices with stems pointed in the opposite directions.