Keychange in only one instrument does not work

Hi there,
because I added a change of an instrument, I need a new key in only that staff. Here is the handbook:

https://musescore.org/en/handbook/3/key-signatures#add-key-sig-to-one-s…
https://musescore.org/de/handbuch/tonarten#add-key-sig-to-one-staff

The problem: It doesn't work that way. If I drag the key signature onto my measure while pressing "cmd" (on Mac), every staff gets the key change. I tried cmd and alt and Shift and ctrl and every combination - without results.

Has anybody an idea?

Thanks and best wishes,
theBore