To be clear - the Instruments dialog is where you add a new instrument (eg, if you have a score for organ only, you would use this to add a staff for trumpet), also to add a new staff for an existing instrument (eg, to change the organ from two staves per syste to three). To simply add measures - which will indeed wrap around to new line as necessary - use Add / Measures.
Comments
Press I to add an instrument (or Edit / Instruments ...);
https://musescore.org/en/handbook/create-new-score-0#add-staff-to-an-in…
Or you mean
To be clear - the Instruments dialog is where you add a new instrument (eg, if you have a score for organ only, you would use this to add a staff for trumpet), also to add a new staff for an existing instrument (eg, to change the organ from two staves per syste to three). To simply add measures - which will indeed wrap around to new line as necessary - use Add / Measures.