https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.sitemapdatasource.startingnodeurl?view=netframework-4.8.1
SiteMapDataSource.StartingNodeUrl Property (System.Web.UI.WebControls) | Microsoft Learn
Gets or sets a node in the site map that the data source then uses as a reference point to retrieve nodes from a hierarchical site map.
property systemweb uimicrosoftlearn