Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
N5EPX-M3J06-TT#LV ;
Just replace the # with the missing letter.
P.S. If you aren't in time to take this single key and you really want it, remember that there's still a giveaway for it. I'll keep the rules and won't post any links, but it's easy to find.
Good luck! :)