so how do I create a leaderboard for that game and also do I need to wait for the game to be developed first or can I just go ahead and create a new leaderboard?

He knows that the game isn't out. He's asking if a board can be made for it, even though it hasn't been released, which can't be done.

In certain high-profile cases a game request might be accepted before release, if that release is imminent.

For a game that may or may not be made with a potential release in the indeterminate but certainly not anywhere near future? No.