I am feeling pretty stupid because surely it can’t be that hard to do.
What do you mean by Bb? Are you trying to place it in the melody, or as a chord? If you’re trying to place a chord, what sort of chord quality is it (major, minor, 7th, suspended, diminished, etc.)?
If it’s in the melody, you just place down a B note, select it, then press the comma "," key to lower a half step. Or you can do the same with A, then pressing period "." to raise it a half step.