If it works similar to the way the amazon.com preorders did, Xbox Live should send you a message before the Beta starts (no ETA on what that date is yet) and you'll be able to enter the code from your messages on Xbox Live or on the Xbox.com website (under "Redeem Code").
You are only to input a code with a game if its a bundle pack or something like a DLC. If you have pre ordered it then I am sure it will come once its installed / able to be given at the given time period.