When you get a beta key...
      

  Unless you're actively seeking a beta invite through various promotions, contests, etc. or have attended BlizzCon, then your only chance is going to be getting an invite through the Battle.net beta opt-in.If selected through the beta opt-in there's no key involved, and beta access is automagically added to your Battle.net account. You'll receive ..
      

http://www.blizzblues.com/us/when-you-get-a-beta-key-27800559991.html