The wiki page says
Liking the Killing Floor 2 Community Page grants players Oisten Jägerhorn as a playable character.
But that page contains no "liking" interface. I see a "follow", or a "subscribe", that to me are not "liking" the page. Moreover, I have tried both, but the character would not be available when launching the game.
What exactly one has to do to unlock this character?