A discount code can only be redeemed when a premium membership has been issued.
After you have logged in and completed the registration process, you are then able to choose your payment method.
If you have a discount code, you can redeem it here. After entering the discount code in the box provided, click "Check discount".
If a green message appears, the code is still valid and you can then proceed with the registration process.
The discount will now appear in your contract which allows you to receive a discounted membership.