Hi - You just have to be a USAA member to get the $1250. If you have a member ID you can call USAA and get a voucher which you provide to the dealer.
I clicked on your link and it says to enter a ID code. I can't figure out how to actually register for the 3 series test drive to get the $1K. I already signed a contract so I'm not even sure if I can apply this now.