If you'd like to add a coupon to an existing subscription:
1. Login to your account
2. Click on 'Manage Subscriptions' on the left hand side
3. Click on 'Apply Discount Code' link
4. Enter your coupon code and click 'Save'
5. Coupon code is now applied!
Of note: The system only allows 1 coupon code to be used at a time, so if your subscription currently has a coupon code attached to it, you will need to 'Delete' that, and then enter and save the new coupon code.
If you're adding a temporary, 1 time coupon code and then wish to revert back to a recurring coupon, login to your account after the 1 time coupon code and corresponding order has shipped, delete that 1 time coupon code and then substitute your recurring one back in. Be sure to hit 'Save' so all processes correctly.