In order to use a promotional code, you need to start a booking process.
After entering your (1) travel information in the simulation module on the homepage, click on (2) "Book" and then (3) "Continue".
Once on the booking page, you will find a box (4) "PROMO CODE" in which you can enter your code.
If it is valid, the field will turn green and your discount will be immediately reflected in the summary on the right of your computer screen.