I brought this up before a while back. When will we be able to select from the new Musical Instruments from the Sword Coast Adventurer Guide from Backgrounds/Starting Equipment? When I take a musical instrument, I'd like to take Proficiency with the Yarting. You can choose/buy the instrument but can't take Proficiency with the instrument at this time.

You can add it as a custom “tool” proficiency.  Tap at the bottom of the “Proficiencies and languages” box (lower left of character sheet) and a window/sheet should pop up that allows you to add proficiencies and languages.

It would still be ideal if they’d add them to the official list, but this will work until they get around to it.  It would also help if they had  “manage proficiencies” text in that box like they do in some other places; it’s not evident that there are custom options in this box.