Enharmonic Key signatures

I play a B Flat trumpet which is a transposing instrument. As part of my practice I sometimes place some of the notes into Musescore and then play the passage to ensure I am playing the correct notes and timing.

I currently have a trumpet exercise written in G flat Major. Because B Flat trumpet is a transposing instrument I have to put the concert pitch into the key signature. The concert key that will transpose into G Flat Major is E Major. Unfortunately when the key is revealed, it shows the key signature as F sharp Major rather than G Flat Major. Whilst F sharp and G Flat are enharmonically the same note, I want it to show as 6 flats rather then 6 sharps. (I expect a similar result will occur for C Flat Major). Does anyone know how to correctly show this in 6 Flats.