You need to cast to your class where you got your function defined
void AMyPlayerController::KeyPressed()
{
AMyCharacter* const Character = (AMyCharacter*)GetCharacter();
Character->DoStuff();
}
You could also add checks if GetCharacter() is indeed returning your class with if(GetCharacter()->IsA(AMyCharacter::StaticClass()))
as wrong cast will crash the game, but if you 100% sure that playercontroller will use only this class then you don’t need to.