And here is the way to get x-default added (why do they make this stuff to hard to find?): Go to the link below and see 'Specifying the default language (x-default)' section...https://www.aem.live/developer/sitemap

Adding this for those who are trying to add <lastmod> date. Instructions are on this page about 3/4 of the way down:https://experienceleague.adobe.com/docs/experience-manager-65/managing/managing-further-reference/seo-and-url-management.html?lang=en