Apple Sharing - Apple IDs

My husband has an Apple ID (@mac.com) that logs into all his devices, however his iTunes acct has a different login ID (yahoo.com) (from way back in the day before we converted to a Mac household.)

He is logged into our AppleTV with his iTunes (yahoo) account. We just upgraded to AppleTV+ and I want to be added to family sharing from his iTunes account so I can watch AppleTV+ on my devices.

Im already on family sharing under his Apple ID. We can’t find a way to add me to family sharing on his iTunes account because it’s not an AppleID that we can log onto on any device & go to system preferences.

I’m stuck, any help would be appreciated!