counterpoint

This music is set up with a single stave, which is configured to have instrument = piano. It can have up to 4 voices per stave.

Note 1: This 'gear' brings up the customize toolbar options. Scroll to the bottom, you'll see quarter notes labeled 3 & 4. They probably have (to the left of themselves) what is "eye closed" (eyelashes). That means "not visible". Click on the eyelashes for 3 & 4, they change to an "eye open", and appear next to notes labeled 1 & 2.

Note 4: Your Musescore 4 window should have this area visible along the bottom-left. Let's call it the "info bar". When you click on most items IN your score, that area will tell you about the thing you clicked on. I clicked on the 2nd (lower) half-note-rest in measure 80 (it shows up green BTW) -- see Note 3. That is a Voice 2 item. Were I to have clicked on the upper half-note in that measure, the info bar would have shown it to be assigned to Voice 1.

Note 2: In measure 70, there are no additional rests, so we're confident that measure only has one voice, which happens to be Voice 1.

See also this Musescore 3 handbook page about note entry & voices. It should apply to Musescore 4:
https://musescore.org/en/handbook/3/voices

I'm not familiar with assigning instrument sounds to VOICES (if such can be done, actually). Somebody else will have to advise on this.

Note 5: Click on 'View', from the drop-down list of menu items, choose 'Instruments'. Click on the gear next to the blank space (Note 6), where you can 1) NAME the instrument used in that stave (presently it is not named), PLUS you can 2) REPLACE the current instrument (now Piano) with the Trumpet you want.