How do I embed tabtip.exe inside windows

I'm trying to embed the osk in a wpf window or a user control and I've found the code below and it's working for notepad but for tabtip.exe, it's saying that it doesn't have a graphical interface??

WaitForInputIdle failed. This could be because the process does not have a graphical interface.

I tried letting it sleep for awhile instead of calling waitForInputIdle method but it throws another exception:

Process has exited, so the requested information is not available.

But in my task manager, I can still see TabTip.exe running.

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private System.Windows.Forms.Panel _panel;
        private Process _process;

        public MainWindow()
        {
            InitializeComponent();
            _panel = new System.Windows.Forms.Panel();
            windowsFormsHost1.Child = _panel;
        }

        [DllImport("user32.dll")]
        private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

        [DllImport("user32.dll", SetLastError = true)]
        private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

        [DllImport("user32")]
        private static extern IntPtr SetParent(IntPtr hWnd, IntPtr hWndParent);

        [DllImport("user32")]
        private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);

        private const int SWP_NOZORDER = 0x0004;
        private const int SWP_NOACTIVATE = 0x0010;
        private const int GWL_STYLE = -16;
        private const int WS_CAPTION = 0x00C00000;
        private const int WS_THICKFRAME = 0x00040000;


        protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
        {
            base.OnClosing(e);
            if (_process != null)
            {
                _process.Refresh();
                _process.Close();
            }
        }

        private void ResizeEmbeddedApp()
        {
            if (_process == null)
                return;

            SetWindowPos(_process.MainWindowHandle, IntPtr.Zero, 0, 0, (int)_panel.ClientSize.Width, (int)_panel.ClientSize.Height, SWP_NOZORDER | SWP_NOACTIVATE);
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            Size size = base.MeasureOverride(availableSize);
            ResizeEmbeddedApp();
            return size;
        }

        private void button1_Click_1(object sender, RoutedEventArgs e)
        {
            button1.Visibility = Visibility.Hidden;
            ProcessStartInfo psi = new ProcessStartInfo("C:\\Program Files\\Common Files\\microsoft shared\\ink\\TabTip.exe");
            _process = Process.Start(psi);

            Thread.Sleep(500);
            //_process.WaitForInputIdle();

            SetParent(_process.MainWindowHandle, _panel.Handle);

            // remove control box
            int style = GetWindowLong(_process.MainWindowHandle, GWL_STYLE);
            style = style & ~WS_CAPTION & ~WS_THICKFRAME;
            SetWindowLong(_process.MainWindowHandle, GWL_STYLE, style);

            // resize embedded application & refresh
            ResizeEmbeddedApp();
        }

    }
}

Edit: Inspired by rene's comment, I've tried to obtain the window ptr as below and used spy++ to verify that the address that FindWindow gives is pointing to the correct window, but it's still not moving:

 IntPtr KeyboardWnd = FindWindow("IPTip_Main_Window", null);


 int style = GetWindowLong(KeyboardWnd, GWL_STYLE);
 style = style & ~WS_CAPTION & ~WS_THICKFRAME;
 SetWindowLong(KeyboardWnd, GWL_STYLE, style);
 SetWindowPos(KeyboardWnd, IntPtr.Zero, 0, 0, (int)_panel.ClientSize.Width, (int)_panel.ClientSize.Height, SWP_NOZORDER | SWP_NOACTIVATE);

Edit 2: My first thought was that tab tip couldn't be resized, but then I noticed a behavior when I try to move the window across two different screen, it'll resize to fit the screen size, so I'm sure there must be a way to resize, so I started spy++(x64) to check :

Edit 3: after tinkering abit with user32 api and no progress, I've tried to use a memory scanner to scan for the x and y position of tabtip and change it, however, it's not refreshing until a repaint is triggered, I'm wondering the feasibility going down that path.

Can you try to run your handle code in STA thread? I had a similar issue with native window, which I had resolved using STA thread.

var thread = new Thread(() => {
          // Your code here
});
thread.TrySetApartmentState(ApartmentState.STA);
thread.Start();