First, HBO should update their instructions because no where does it mention to append anything to your username.
Second, why pre-select a provider?
These aren't questions for you but anomalies with the app. However, it appears the OP has successfully logged in with other platforms (Mac & Contour) without appending domains. If a domain was required, the OP would have suspected it as the problem.
Contour wouldn't require it, being a cox product, it would automatically assume an @cox.net, by adding it to the username if it didn't already have it.
When you log in, you're logging into the HBO Max app...not Contour. The app sends the request to Cox. HBO wouldn't allow the programmers of Contour to access the scripting of its app to append data. If Cox requested HBO to append the domain of a username on its Contour, this would be HBO scripting the app to append the domain.
For a mobile device, if HBO can append the domain of a provider on a cable-box, why couldn't HBO script its app to append the domain of a provider after a user selects said provider in the app?
Because, good question, ask HBO. When I had to troubleshoot issues with their app, over HALF of the issues were resolved by adding @domain.xxx to the username. They do NOT document that well, and could automatically add the domain when you select a provider, but, they don't/////
