Mailgun\Messages\MessageBuilder::addCustomParameter PHP Method

    public function addCustomParameter($parameterName, $data)
    {
        if (isset($this->message[$parameterName])) {
            array_push($this->message[$parameterName], $data);
            return $this->message[$parameterName];
        } else {
            $this->message[$parameterName] = [$data];
            return $this->message[$parameterName];
        }
    }