Mailgun\Messages\MessageBuilder::addCustomData PHP Method

    public function addCustomData($customName, $data)
    {
        $this->message['v:' . $customName] = json_encode($data);
    }

 /**
  * Attach custom data to a message.
  *
  * @param string $key
  * @param mixed  $data
  *
  * @return \Bogardo\Mailgun\Mail\Message
  */
 public function data($key, $data)
 {
     $this->messageBuilder->addCustomData($key, $data);
     return $this;
 }