If you're here, you have an Apple TV, you've signed into your cable service, and you're trying to connect to HBO Max. You can't use your cable company (probably something like firstname.lastname@example.org) and its password. When you try to sign in using your provider, you see the following message:
Complete Your HBO Max Registration
Please do one of the following
Phone or tablet: Sign in to the HBO Max app Computer: Sign in to HBOMAX.com on your computer
Pictured: The dreaded complete your HBO Max registration error
Below is an OK button. Nothing else is explained, and of course, HBO's documentation sucks and doesn't help.
First, use a computer, and sign in to your HBO Max account using your cable login. Next, go to your profile. Click in the upper right-hand corner and click account.
Pictured: Account window in the HBO Max website.
Make sure you have an email address here and that it's verified. You'll have to work through the verification process involving emails if it isn't. Then make sure you know the password in this section, as this will be what you're after.
Once you have verified your account, you can now go back to your Apple TV. Instead of using sign-in with your cable provider in the HBO Max app, you will use your login information from this panel (The email address as your login and the password is the password).
I also noticed that after the sign-in, it goofed, and it didn't seem to work. I quit the app, and it loaded again, this time presenting me with a profile choice, and HBO Max was working.
Hopefully, this really stupid fix helps. I wasted about an hour trying to figure out what was wrong only to discover this was the issue.