You can put your 9 digit beta code into the bnet site now, it reserves your spot and let's bungie know you're ready for action. Then on the 10th, when the site is updated(yay!) you'll be able to choose platform and region for the beta. Then when the beta actually starts you'll be sent a key that unlocks the beta download on your platform. But yes, should be able to put your code in at anytime leading up to the beta. Confusing, but hope that answers your question