You can add a promo code to an existing recurring payment by completing the following steps:
Click on the recurring payment from your home screen
Click the "More" button
Click "Enter Promo Code"
Enter your promo code, then click "Apply"
You cannot remove a promo code that has been applied to a payment.
Pausing a payment keeps the promo on that payment for later should you decide to activate it again.
You cannot add a promo code to a recurring payment that is in progress or a one time payment.