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.
DN5#4-TJNTV-ZLART ;
Just replace the # with the missing letter.
A leftover key which I decided to share here. Don't be upset if you're not in time with getting it, since this game has a nice discount on Steam now :)
Good luck and stay safe!