menu.KeyboardNavigationSettings.CommandKey = KeyboardNavigationModifier.Ctrl;
            menu.KeyboardNavigationSettings.FocusKey= KeyboardNavigationKey.M;
            menu.EnableAriaSupport = true;

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        // Create the RadMenu control
        RadMenu menu = new RadMenu();
        menu.ID = "RadMenu1";
        menu.Skin = "Default";
        menu.OnClientItemFocus = "OnClientItemFocus";
        menu.OnClientItemClosing = "OnClientItemClosing";

        // Create menu items
        RadMenuItem item1 = new RadMenuItem("Item 1");
        RadMenuItem item2 = new RadMenuItem("Item 2");
        RadMenuItem item3 = new RadMenuItem("Item 3");

        // Add sub-items to Item 1
        RadMenuItem subItem1 = new RadMenuItem("Sub Item 1");
        RadMenuItem subItem2 = new RadMenuItem("Sub Item 2");
        item1.Items.Add(subItem1);
        item1.Items.Add(subItem2);

        // Add items to the menu
        menu.Items.Add(item1);
        menu.Items.Add(item2);
        menu.Items.Add(item3);

        // Add the menu to the page
        // Assuming you have a placeholder control named "menuPlaceholder" in your markup
        menuPlaceholder.Controls.Add(menu);

        // Register the startup script
        string script = @"
            function OnClientItemFocus(sender, eventArgs) {
                var item = eventArgs.get_item();
                if (item) {
                    item.open();
                    // document.getElementById('textbox1').focus();
                    $telerik.cancelRawEvent(eventArgs);
                    return false;
                }
            }

            function OnClientItemClosing(sender, args) {
                args.set_cancel(true);
            }
        ";

        ScriptManager.RegisterStartupScript(this, this.GetType(), "CustomScript", script, true);
    }
}

            <script>
                function OnClientItemFocus(sender, eventArgs) {
                    var item = eventArgs.get_item();
                    if (item) {
                        item.open();
                        // document.getElementById("textbox1").focus();
                        $telerik.cancelRawEvent(eventArgs);
                        return false;
                    }
                }

                function OnClientItemClosing(sender, args) {
                    args.set_cancel(true);
                }
            </script>
            <telerik:RadMenu runat="server" ID="RadMenu1" Skin="Default"  OnClientItemFocus="OnClientItemFocus" OnClientItemClosing="OnClientItemClosing" ...