Adding RTA label

Hi all! I would like to add the RTA (Restricted To Adults) label <meta name="RATING" content="RTA-5042-1996-1400-1577-RTA" /> to my website to tag it as adult content. But I cannot find any place where to add custom meta information. How could I do that with this module? At present I added it to my custom theme html.tpl.php template but I would prefer the Metatags module way.