As a Roblox developer, it is currently impossible to promote a new game on Xbox. The thread in which you can submit Xbox builds for your game is not kept up to date and it seems submissions are being ignored.
I have spent a fair amount of time creating a new game and ensuring that it is playable on Xbox, even buying an Xbox to test. On other platforms you can buy sponsored ads to draw new players, but there are no sponsored ads for console.
The handful of players who stumbled upon my game tend to play for long periods of time, but I have no way of drawing more players.