lib/Api/Subscription.php
public function associateCouponToASubscription(string $subscriptionId, string $couponCode)
    {
        $response = $this->sendRequest('POST', sprintf('subscriptions/%s/coupons/%s', $subscriptionId, $couponCode));

        return $this->processResponse($response);
    }
public function associateCouponToASubscription(string $subscriptionId, string $couponCode)
    {
        $response = $this->sendRequest('POST', sprintf('subscriptions/%s/coupons/%s', $subscriptionId, $couponCode));

        return $this->processResponse($response);
    }
@param string $subscriptionId The subscription's id
@param string $couponCode     The coupon's code

@throws \Exception

@return array