Добавляем свои кнопки в заголовок окна WPF(WinAPI)

using System;
using System.Windows;
using System.Runtime.InteropServices;
using System.Windows.Interop;

namespace WpfApplication3
{    
    // Interaction logic for Window1.xaml
    
    public partial class Window1 : Window
    {
        #region Win32 API Stuff

        // Define the Win32 API methods we are going to use
        [DllImport("user32.dll")]
        private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

        [DllImport("user32.dll")]
        private static extern bool InsertMenu(IntPtr hMenu, Int32 wPosition, Int32 wFlags, Int32 wIDNewItem, string lpNewItem);

        /// Define our Constants we will use
        public const Int32 WM_SYSCOMMAND = 0x112;
        public const Int32 MF_SEPARATOR = 0x800;
        public const Int32 MF_BYPOSITION = 0x400;
        public const Int32 MF_STRING = 0x0;

        #endregion
        
        public const Int32 _SettingsSysMenuID = 1000;
        public const Int32 _AboutSysMenuID = 1001;
        
        public IntPtr Handle
        {
            get
            {
                return new WindowInteropHelper(this).Handle;
            }
        }

        public Window1()
        {
            InitializeComponent();

            this.Loaded += new RoutedEventHandler(Window1_Loaded);
        }

        private void Window1_Loaded(object sender, RoutedEventArgs e)
        {
           
            IntPtr systemMenuHandle = GetSystemMenu(this.Handle, false);

            
            InsertMenu(systemMenuHandle, 5, MF_BYPOSITION | MF_SEPARATOR, 0, string.Empty); 
            InsertMenu(systemMenuHandle, 6, MF_BYPOSITION, _SettingsSysMenuID, "Settings...");
            InsertMenu(systemMenuHandle, 7, MF_BYPOSITION, _AboutSysMenuID, "About...");

            // Attach our WndProc handler to this Window
            HwndSource source = HwndSource.FromHwnd(this.Handle);
            source.AddHook(new HwndSourceHook(WndProc));
        }

        private static IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
        {            
            if (msg == WM_SYSCOMMAND)
            {
                // Execute the appropriate code for the System Menu item that was clicked
                switch (wParam.ToInt32())
                {
                    case _SettingsSysMenuID:
                        MessageBox.Show("Settings");
                        handled = true;
                        break;
                    case _AboutSysMenuID:
                        MessageBox.Show("About");
                        handled = true;
                        break;
                }
            }

            return IntPtr.Zero;
        }
    }
}

Вот, что получилось у меня: