I just signed up for YouTube TV and had zero issues adding it as a source. I have the server installed on a Mac Mini 2 and set it up on my iPhone. I have 2FA turned on and when I plugged in my credentials the first time it failed. I did it again and opened the YouTube app on my iPhone to authenticate the sign in as soon as I put my credentials into Channels again. It allowed me to approve the sign in within the YouTube app and started scanning my channels.
I hope this helps!