How to alter a trill

To be honest, you can't really make an altered trill in MuseScore out of thin air. The best choice is to make it yourself.

What you should do is write the note that will be trilled (simple whole note as an example), then put a trill symbol above it and add the accidental symbol next to the trill symbol. Next, go into the inspector and mute the whole note so that it wont be played.

Now, in the same measure, add a new set of notes but in the 2nd voice. These notes can be 32nd notes so the trill will sound like a trill (in your case, these notes will trill from d# to e natural). Afterwards, go into inspector and make the notes invisible (it is a LOT of work to make those invisible, but it's worth it in the end).

Then, when you're all done, just listen to the playback. It's a lot of work just to make an altered trill, but it's the best route to take.

Listen to this score to see what the final result should look and sound like (skip to 4:05).
https://musescore.com/user/31483852/scores/7981508