code: public function associateCouponToASubscription(string $subscriptionId, string $couponCode)
      {
          $response = $this->sendRequest('POST', sprintf('subscriptions/%s/coupons/%s', $subscriptionId, $couponCode));
  
          return $this->processResponse($response);
      }
docs: @param string $subscriptionId The subscription's id
  @param string $couponCode     The coupon's code
  
  @throws \Exception
  
  @return array