how do I get it

wait for the store to be back up and running.

did you save your receipt? Or does your GameStop you ordered from have your email? GameStop/EB said that they were going to contact the pre-order folks once the store comes back up..

You can also go on the Brink website, there is a spot that you can enter the number on your receipt to reserve your pre-order code and they will Email you when the codes are active..

There is another thread that has the link..