I had the same problem, but I managed to do it. but before I tell you how, I feel you need to be warned about doing it.
I managed to link the accounts no issues with my account at all, but the real problems started when trying to actually run the game through steam. it's a very, very, bad idea to do so right now. As when I did, the game absolutely refuses to launch at all. once i started the launcher the steam client starts ticking away that I'm playing, but in actuality I cant get past the first launcher, as the launcher immediately crashed upon clicking start game.
I resolved that as well, but it took a total wipe of my system and a reinstall fresh to do it. And so, now I have the account inked, but it will not actually run the launcher. I've found a way to make both work without affecting the other, but I highly recommend, NOT using steam to play.
So in the end I have a steam linked account, which I actually cannot use. But if you still want to do it, you need to launch steam, go to the little menu bar up top under games there will be an option to activate a product, use that to redeem the steam code they give you.