Do I have to get legal permission to advertise that my game is on Xbox consoles?

Probably, but you should ask Roblox this, since you've created something for their game, what their rules for advertising are

Just don't use the copyrighted logos if you're worried. The only reason why you see a single carrier name in game advertisements is because they paid for co-branding

never played xbox roblox, so idk if it has ads, but u should be allowed to since roblox IS on xbox.

I currently do not own an Xbox, I am trying to do this to get adult permission for Fable 3 as I am 16 (Legal age in Australia for MA15+)

The only way you're going to get the demo downloaded is if you have a parent or adult sign in and download it on their profile. It's a legal restriction built in to prevent minors from viewing adult advertisement, which a demo is. Your parent or guardian
can essentially give you permission to view it... by downloading it on their profile.

really, everyone does it. I'm not advertising, I'm trying to help new players join a active clan so they can get help in the game with builds, weapons and leveling up. If that is the case I'm done with the game and xbox. I'm following thru with legal action with xbox. I have the money to do it.