Evolve alpha, which this Euro reporter sadly notes is limited to North America.
Personal grievances aside, here's what you need to do for a chance of being selected. Head to the sign-up site, and when asked enter the password "happyhunting" and referral code "JoinTheHunt" along with all required details. Also, make sure you have a Steam account as you'll need one. 2K Games aims to have its participant list nailed down by tomorrow, July 5, so don't procrastinate before you celebrate today.