The best way folks know to get on the list is to sign up at the Disney website, make sure and check the box that says something about "send me info/promotional stuff", and then plan a few trips and save them. Do not BOOK them, just save a few different scenarios... like different travel times, different resorts, etc.
Tweak them a few times, and several people have said that a couple of weeks later they get an email from Disney Destinations with a PIN. Be sure and pick the time of year you are mostly interested in, because you don't want a PIN that isn't for your dates!