Try the following sample code :
MenuItem rootitem = new MenuItem("Ariticles");
MenuItem childItem = new MenuItem("Ariticle 1");
MenuItem childItem2 = new MenuItem("Ariticle 2");
childItem.NavigateUrl = "sample.aspx";
childItem2.NavigateUrl = "sample2.aspx";
rootitem.ChildItems.Add(childItem);
rootitem.ChildItems.Add(childItem2);
Menu1.Items.Add(rootitem);
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