Sign in to your account and go to the "Account Settings" =>"Promo codes" and copy and paste your promo code, 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: a promocode can be applied only before making the payment.
There is no option to apply a promo code after the payment has been made nor to save a store credit at this time. All promo codes are for a single-time use only.