Sign in to your account and go to the "Account Settings" =>"Promo codes" and copy and paste your promo code, then click "Apply".
After the code is redeemed, click "Try PRO" and you will see the discounted price 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.
*promo codes do not apply to the subscription renewals