If you want to add a coupon to an existing subscription, it's quite easy to do whether you want to do it to a single individual or a group of subscribers.
Keep in mind that this coupon will not be applied immediately and will only be applied upon the next renewal.
For Individual Action
Step 1: Go to Subscriptions and Find the Subscription
From the merchant portal, choose Subscriptions from the navigation menu.
You can use the search bar to search by customer name or email or the id of the subscription.
Step 2: Click on Action Menu in the Upper-right
Once you've clicked on the Action menu, you should see a drop-down with the option to "Add a Coupon." Choose "Add a Coupon."
Step 3: Choose Your Coupon and Confirm It
From the dropdown find the coupon you would like to apply to the subscription. Once you've chosen the coupon, click "add coupon" then select "apply to next renewal".
For Group Action
Want to apply a coupon to more than 20 subscriptions? Please create a ticket and send a CSV file with the subscription_ids, coupon_id, and coupon code to Cratejoy support.