Accordion counterbass fingering symbol

In the attachment I successfully added a small dash (a tenuto) above the notehead to indicate the counterbass is to be played. (Standard Accordion notation) The fingering of those counterbass notes also needs a little black line displayed beneath them (as shown by the red dash I added in Paint). How can I use MuseScore to add those (black) lines below the fingers?

Thanks,
fingers.