right after u finish ur tutorial...
Simply because u will get 100k WHICH is very important useage for your enhancing as it will cost u greatly later in game....
The code will also provide u a rare card to get a good start in game
Helping you to fight boss after every 5 part of a chapter in mission.
In addition cards will help u bring down other players for you to earn extra golds and treasures to get more cards!
So what are you waiting for?
Tips: for perfect enhance get 8 of the same card (base form) and max enhanced every single of it.