public function start_on_drip_campaign($recipient_address, $drip_campaign_id, $data=null, $args=null){
        $endpoint = "drip_campaigns/" . $drip_campaign_id . "/activate";

        $payload = array();
        if (is_array($recipient_address)) {
            $payload["recipient"] = $recipient_address;
        } else if (is_string($recipient_address)){
            $payload = array(
                "recipient_address" => $recipient_address
            );
        }

        if (is_array($data)) {
            $payload['email_data'] = $data;
        }

        if (is_array($args)) {
            $payload = array_merge($args, $payload);
        }

        return $this->api_request($endpoint, self::HTTP_POST, $payload);
    }