I've moved this to the Youtube forum.
Hi,
I've just tried this on the Youtube app for Amazon firestick and I guess the process is similar for roku. You'll need to be in front of your TV and also in front of a PC connected to the internet.
(A brand account is attached to a google account which in turn has its unique email address. You'll need that email address and password to that google account.)
Here is what I had to do for Amazon firestick:
1)In the YT app on firestick, click on "add account". It asks me for email address (type in email address for the google account that your brand account is attached to) and then enter the password for that same google account.
2)The app (on Amazon TV) then displayed a code on the screen and told me to open a browser, go to this address: youtube.com/activate and then enter the code. I did that on a PC.
3)Still on the PC, it asked me to select the google account and email address that is attached to the Youtube brand account I'm trying to log in for for. I select it in the list.
4) Still on my PC - It then asks me which YouTube account/channel I want to login with. I have a few channels which are attached to that google account so I choose the channel I want and click save/confirm.
5)Back on the TV, the account/channel is now logged on and added to the list for future use.
hth