At the bottom of this page on their site. Isn’t super clear, but they make a distinct break in the page for the different retailers

It looks like if you “pre-purchase” it through Ubisoft, Xbox Marketplace, PSN, or Steam you get access to the Stick of Truth immediately. If you preorder it through retailers that you don’t pay the whole thing up front (Gamestop, Amazon, etc) it looks like you get it when you pick up the new game. That’s how I’m…