Here's what it says after you order:
Coupon code will be sent to email address associated with purchase within 10 days of game being shipped. Please check your junk/spam mail.
How to redeem your promo code:
To get up to your free Xbox LIVE 1600 pts from Microsoft Store online, just return to the store, place the Xbox LIVE 1600 pts product in your cart, hit the “Checkout” button. You’ll see a place to enter your promo code, just enter the coupon code we will email to you after the game is shipped and hit “Apply.” Your discount will then be applied to your order.
Valid for single use only. Does not apply to value of any taxes, shipping and handling. Only valid online at the US Microsoft Store website: www.microsoftstore.com. Cannot be transferred, redeemed for cash or combined with any other offer or promotion. Expires 12/31/2013.