The FAQ says:
QuoteIf you also played TERA on Steam, you will be asked whether you want to migrate on Steam as well. Please note that account linking between the Gameforge Client and Steam will only continue to work by successfully migrating on Steam as well.
But I can't figure out how to link the client and Steam. I've installed the game through Steam, and it works, but now, if it want to start the game through the client, it wants me to install the game again. I'm assuming if I link them, the client will know the game is already installed?