private function createPostFields(Notification $notification)
    {
        /**
         * Required fields
         */
        $data = [
            'user_credentials'           => $this->accessToken,
            'notification[title]'        => $notification->getTitle(),
            'notification[long_message]' => $notification->getContent()
        ];

        /**
         * Optional fields
         */
        if (!empty($this->getSourceName())) {
            $data['notification[source_name]'] = $this->getSourceName();
        }

        if (!empty($this->getIconUrl())) {
            $data['notification[icon_url]'] = $this->getIconUrl();
        }

        if (!empty($this->getSound())) {
            $data['notification[sound]'] = $this->getSound();
        }

        if (!empty($this->getOpenUrl())) {
            $data['notification[open_url]'] = $this->getOpenUrl();
        }

        return $data;
    }