You can redeem a bonus code you have received by logging into your player account and opening the menu. You will find an input field with a gift symbol at the bottom left. Enter the bonus code you received from 7 Melons and click on "confirm".
The bonus will be credited to your account immediately. You will find it on the home page of your account. There you can view the progress of the turnover requirements or remove the bonus if necessary.
Important: A cancelled bonus cannot be restored. If you cancel an active bonus, you will not only lose your previous playthrough progress and the bonus balance, but also the right to this bonus.