Going by the image, Microsoft's dealing out the promo to some of its "very best customers." As can often prove the case with offers like this, however, the criteria for who qualifies is seemingly random. For example, it's fair say the Joystiq staff's spent oodles of times with their 360s, and none of us received the offer. As Eurogamer notes, the original NeoGAF poster got the promo despite already owning an Xbox One tied to his Xbox 360 gamertag.
The conditions state the code arrives on August 15 and must be redeemed by October 15. After that, there's a year starting at the point of redemption to spend that Xbox credit. If you're one of the lucky ones wondering how to spend that moolah, you could do a lot worse than starting with emotional UbiArt downloadable Valiant Hearts: The Great War.