SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://www.progress.com/documentation/sitefinity-cms/add-a-custom-url-to-the-sitemap

Sitemap_Urls_Global.asax.cs

using System;
using System.Linq;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.SitemapGenerator.Abstractions.Events;
using Telerik.Sitefinity.SitemapGenerator.Data;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {

        protected void Application_Start(object sender, EventArgs e)
        {
            Bootstrapper.Initialized += new EventHandler<Telerik.Sitefinity.Data.ExecutedEventArgs>(this.Bootstrapper_Initialized);
        }

        void Bootstrapper_Initialized(object sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
        {
            if (e.CommandName == "Bootstrapped")
            {
                EventHub.Subscribe<ISitemapGeneratorBeforeWriting>(Before_Writing);
            }
        }

        private void Before_Writing(ISitemapGeneratorBeforeWriting evt)
        {
            // gets the entries that are about to be written in the sitemap
            var entries = evt.Entries.ToList();

            // creates a new sitemap entry
            SitemapEntry newSitemapEntry = new SitemapEntry();

            //sets the location property of the new sitemap entry
            newSitemapEntry.Location = "http://<site domain>/my-custom-entry";
            newSitemapEntry.Priority = 1;

            // adds the new sitemap entry to the collection of the entries
            entries.Add(newSitemapEntry);

            // sets the collection of entries to the modified collection
            evt.Entries = entries;
        }
    }
}