The enharmonic preference of the key is determined by the key that you transpose the song into. For instance, if you pick the key of F#, then F# will be used. If you pick the key of Gb, then Gb will be used, etc. You can choose from more key options by turning on theoretical keys. To do this, go into the
Utilities Menu by tapping on the gear icon in the
Menubar and then choose
Settings >
Menu Settings >
Style Preferences >
Chords >
Key >
Theoretical Keys and turn that on. This will add an additional row of key signatures you can choose from in the
Style Preferences Menu.
Another option is to turn off transposition. This will allow whatever you've typed to be output as-is without transposition applied. To do this, go into the the
Style Preferences Menu by tapping on the sliders icon in the
Menubar and then choose the
Chords tab. Locate the Key field and tap on the power icon to the right to turn off transposition.