public function addCoupon(string $subscriptionId, string $couponId)
    {
        // Log Debug
        $this->flow->getLogger()->debug("Adding Coupon $couponId to $subscriptionId");

        return $this->make(
            $this->flow->send(
                'post',
                $this->endpoint . '/addCoupon',
                [
                    'subscriptionId' => $subscriptionId,
                    'couponId' => $couponId,
                ]
            )
        );
    }