Accordion Counter Bass

I am a beginner accordion player. I'm using MuseScore to make a copy of the songs/practices that I am learning. Larger print, old eyes. :)
Counter bass notes are on the left hand button keyboard of an accordion. According to the book my instructor has me working out of, "Counter basses are always notated in the music with a dash under the note."
Is there any way I can put a dash under a note? Or is there a way I can add a dash to the Fingering tool?
Thanks in advance.