Don't know if there a topic about this since I'm a few days late, but it seems they started giving out the codes today.
So here's what you do.
Make an account at gameinformer
Visit this link. You must have adblock disabled.
Wait for the email to be sent to your account with the key.
Make a Gamestop PC Download account. Don't worry you'll get a steam key.
Download the Gamestop PC desktop app.
Go to the My Games on the Gamestop PC Tab, click Darksiders and click the "Redeem Key" button on the top right.
Copy and paste that key into Steam and you're done!
Pretty sure it's just for the US or North America.
But hey, pretty sure proxies work.