It seems that the Tree of Savior official site mistakenly leaked an announcement which indicated that the international open beta will start on March 14, 2016. The post was removed immediately but it's captured by player in screenshot.
According to the post players are supposed to be able to preload the OBT client on Steam starting March 7 but nothing happen so far. Still, it's very likely the game will start open beta this month.
Related: What's the Best Class Build for International Open Beta