Hi there. Please see https://forum.atlas-games.com/t/instructions-for-gms/1597/1 for directions on how to set up a new play by post game.
In particular, you need to send an email to "forum AT atlas-games DOT com" requesting that a new forum be opened up for your game (don't forget the dash in the middle). The email should include your username, the title of your campaign, the Atlas Games system you're using, and a brief description of the campaign (just a sentence or two, which we'll use as your forum's subtitle).