I was under the impression the OP had just subscribed. If so, Cox wouldn't have issued an email address because Cox stopped email service to new subscribers as of, I think, 2015.
UPDATE: Not 2015 but 2019.
"Cox Email is a free service that was included with all Cox High Speed Internet packages. As of August 15, 2019, Cox Email is no longer available to new users."
Correct, but you still get a username to login to services and to cox's web site, and services like hbo max require the @cox.net to point their system to where to verify the account. "Hoobedoo" as a username doesn't tell them to check COX for subscription, that's why they have to add the @cox.net even though it's not an email address.
When you set up your account on the app, you need to select Cox as your provider.
You STILL have to use @cox.net as part of the username. Until you are logged in, HBO doesn't know who your provider is.
Did you see OP's image? It has a specific "Cox" screen to log in.
You select Cox as your provider before logging in with your Cox credentials. This informs the HBO Max app of your provider.
