The second wave of keys is now being distributed get them while you can!
You will need to own PUBG to receive a key.
http://mapbeta.playbattlegrounds.com/ where you receive your key.
http://store.steampowered.com/news/externalpost/steam_community_announcements/2758894130294081855 Steam post regarding announcement.
Codename: Savage Testing Round Two on the Closed Experimental Server:
OPENS: Mon, April 16, 6pm PDT / Tues, April 17, 3am CEST / April 17, 10am KST
CLOSES: Wed, April 18, 6pm PDT / Thur, April 19, 3am CEST / April 19, 10am KST