@page "/sitemap.xml"
@using Microsoft.AspNetCore.Http
@{
    var pages = new List<dynamic>
    {
        new {Url = "https://mydarlingunderwear.ru/", LastUpdated = DateTime.Today}
    };
    Layout = null;
    Response.ContentType = "text/xml";
    await Response.WriteAsync("<?xml version='1.0' encoding='UTF-8' ?>");
}

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    @foreach (var p in pages)
    {
        <url>
            <loc>@p.Url</loc>
            <lastmod>@p.LastUpdated.ToString("yyyy-MM-dd")</lastmod>
        </url>
    }
</urlset>