I scoured the website, and but after signing up, I still have no idea how this thing is going to know what my gamertag is. Can I link it somehow? Was I supposed to sign up with my Live ID?
You’re good so far. Just make sure at some point you click on “Dashboard” in the top right (after logging in) and clicking on Xbox Live Sign In (a green bar that should be above the Facebook connect button on the top left of the screen). Hope that helps!
Alright, so you have an account on dewxp.com. Now, click on the button in the top right that says “Dashboard”. On the next screen, there should be a green button on the top left, above your Facebook name, that says something like “Sign in to Xbox Live Here”.
If that isn’t visible, download a different browser. I couldn’t see the button on Google Chrome, I could see the button on Internet Explorer but it didn’t work, and I finally got it to work on Firefox.
Once you sign in there, it should be good to go. However, some people have advised that you not push your DEXP games to the account yet. Something in the official rules states that they can be lost if you haven’t played Halo 4 online yet, which I highly doubt you have 
You can enter the codes, but don’t hit the button that says “Apply”, which is right next to “Gift”, until you have played a game of War Games on November 6th or whenever you get it.
Hope that helps!