Hi @trinhvnguyen the “External Sitemaps” field is meant for entering the URLs of other sitemaps that you wish to add to the index. Not for complete sitemap source code.
Either use the field “External web pages” and enter:
Or create a static file in your site root called (for example) my-sitemap.xml with your sitemap source code in it, then use the “External Sitemaps” to enter:
Hope that helps.