How to break sections part way through a bar?

Hi everyone.
I wonder if anyone can help me please...
I'm transcribing a Brass Band march (copies are seriously old and the print is worn to a point of being almost illegible... plus, a full score wouldn't go astray!), where the Trio begins on quaver beat 3 of a 6/8 bar. There's also the usual key change here, and I'm not sure how to make it work in MuseScore4.
Does anyone have any suggestions please?