I just got home from work, and haven't had the Xbox on in months, so i guess i have system update right now.
But if you have the app on your phone just go to the store and type Game Preview in your search bar its in the list. I bought it this way like 5 mins ago.
Been so excited to get to play this, but i think it'll be a day or two before I get a chance to play. Its gotta download, the most painful part of the process