I think I have found where is hidden info about email bonus code. In Bonus section appeared today (even though I have spent them yesterday) "Total Special Bonus" and "Total Locked Bonus", and by clicking there, there is a formula "Unlocked BCD = Wager/Bet Amount × 1% × unlock rate%". Unlock rate varies from account level.

Now comes most hilarious part, if I click on details there and scroll down, amount that I have received by email code is called "ShitLink Bonus" I am going to suggest their marketing to rename that, because its not good to call their own promotional emails like that.