DC (GC?) kindly shared the link on main (see it above). After winning the flash game you can enter your e-mail and they will mail you a key. You can activate the key on Steam, which will give you a coupon in your inventory. You can try it 10 times.
Here is the list of things you can win (or at least it was when I was checking it, not sure if they change it):
I've won Steam key for Trine Discount Coupon 75%. PM me if anyone needs it.