How to get Sandbox mode running on Apple TV?

I have read this post, but it did not help any, I can't get the sandbox mode working on Apple TV. What I did:

• Create a sandbox user on ITC

• Build the app from Unity as Development Build, "Run in XCode as" Debug

• Added GameCenter capability in XCode (clicking Fix issue on Game Center)

• Ran the standard Authenticate function in code

The popup shows up alright, but first

• there's no ***Sandbox*** everybody is talking about (I haven't seen that even once) and second

• after providing credentials it switches over to the Apple TV GameCenter and then prompts me to configure the account via iTunes, the default for normal accounts.

How am I getting this to work, I have no idea? Many thanks in advance