After downloading the HBO Max app for your iOS or tvOS device, you need to sign in to the app in order to watch HBO Max content
1) Existing HBO for Apple TV Channels subscriber, you select “Already subscribed? Sign In”
2) Select “Sign In Through TV or Mobile Provider”
3) In the list of TV Providers, for HBO subscribers to Apple TV Channels you select “Other TV Providers”
4) Select "Apple TV" as the provider
5) Click Continue to the message HBOMAX wants to use HBO.com to Sign In
6) And then you are finally present with the message to enter your Apple ID to watch HBO Max subscription