The only code you need is the one Bet365 emails you after you've signed up. That's the code you enter to claim your bet credits. You enter it in the relevant place within the 'My Offers' section of your account.
Once you have met the qualifying conditions, you can use your Bet Credits to stake bets without risking your own cash. Bet Credits can be used on any sport across a wide range of markets. Any returns from bets placed with Bet Credits are added to your Withdrawable Balance, returns exclude your Bet Credits stake.
