How do I get my secret code from?
If you are at the following screen, it means that you can credit your friend by letting him add your friend code. It's to let the game know that you play because of a friend. You can't credit yourself for letting yourself into the game.
If this doesn't answer your question, you may want to contact Live Support.
Hope this helps!
I believe Whitie is referring to the (formerly) daily social media code.
The code is posted on the CoL2 twitter feed. They used to post a new one every day at the same time, but for the past couple of months it has been sporadic and at random times.