The release date for Guild Wars 2: Heart of Thorns has yet to be released, but that didn't stop ArenaNet from giving players the chance to get chosen for the upcoming and yet unscheduled beta. ArenaNet will be picking beta testers for their first expansion from a list of active players and "loyal customers who are also newsletter subscribers."
People who are picked to play in the Heart of Thorns closed beta will be notified emailed email, that means that the sign up page for the beta is the newsletter sign up.
If you previously opted out of receiving newsletters, it's time you opt back in. It should be noted that just because you sign up for the beta, that does not guarantee that you will be able a part of the beta test or stress test. Also, If you get into the closed beta for Heart of Thorns, it doesn't mean you will be able to access future beta events.
In order to get in on the beta, you need an active Guild Wars 2 account, which could possibly mean you had to have been actively playing the game recently. If not, you can buy the game for dirt cheap right now. The game is on sale for $10!
You can find the sign up page here, if you're interested.