post
https://secure.fusebill.com/v1/SubscriptionCoupons
This function is used to apply a coupon to a customer's subscription. It is recommended you call the validate coupon call first to ensure this coupon can be applied to the subscription.
Discounts from coupons will not apply to unincluded products.When adding a coupon to a subscription, the discounts will only be applied if the products have already been included. If an invoice is already in the Draft - Ready or Posted state, it is too late for a discount from a coupon to be applied.