Sign in to your account and go to the "Account Settings" =>"Promocodes" and copy and paste your promocode, then click "Activate".
After that, go to the "Subscription" tab, choose the plan type you have the coupon code for and you will see the price with the discount at the checkout:
Note: promo code can be applied before making the payment. There is no option to apply a promocode after the payment has been made nor to save a store credit at this time
All promocodes are for a single-time use only.