The instructions for adding a sitemap are at https://support.google.com/sites/answer/100283?hl=en
This works when first adding a site to the search console, but after adding a site use the following instructions to add a sitemap.
Go to the search console dashboard https://www.google.com/webmasters/tools/home. Is your site listed? Click its listing, click the Sitemaps area.
If it says "No sitemap found" click the "add a sitemap" button.