StackPanel spMain = new StackPanel() { Orientation = Orientation.Vertical };
((StackPanel)FindName("spMain")).Children.Add(spMain);                
//menu
Menu menu = new Menu()
{
    Margin = new Thickness(0, 10, 0, 0),               
    Foreground = Brushes.White,
    Background = new LinearGradientBrush(Colors.Teal, Colors.Honeydew, new Point(0, 0), new Point(1, 1))
};
spMain.Children.Add(menu);  
//menu_items
MenuItem itemMain = new MenuItem()
{
   Header = "Отдел",
   Foreground = Brushes.WhiteSmoke
};
menu.Items.Add(itemMain);
//заполнение подменю