The discount coupon is used at the end of your subscription, on the "Payment" page.
You can access this page by clicking on "Get covered for €X/month" in the top right corner of the summary page of your quote.
Once you have entered the promo code, the discount is applied directly on the amount of your first payment.
How to do it?
In the "Payment information" box, at the bottom, you will find the "Add a promo or referral code" link.
Click on it : a small window will open. Enter your discount code in the blue rectangle, then click on "Add code".
It's done ! Your subscription payment is modified to include the discount, and the benefit earned is displayed under the amount.
There you go 🎉
All you have to do is enter your payment information, click on "Get covered for €XX/month" to make your payment, and continue to the contract signing page.