Send key macro to docked program in winform even if minimized/background

public partial class FormMain : Form
{
    // Interop
    [DllImport("user32.dll")]
    public static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);

    [DllImport("user32.dll", SetLastError = true)]   
    static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);   

    [DllImport("user32.dll")]
    private static extern bool PostMessage(IntPtr hWnd, UInt32 Msg, UInt32 wParam, UInt32 lParam);   

    // Notepad starter
    Process process = null;

    private void buttonStart_Click(object sender, EventArgs e)
    {
        if (process != null)
            return;
        process = new Process();
        try
        {
            string winpath = Environment.GetEnvironmentVariable("windir");
            process.StartInfo.UseShellExecute = false;
            process.StartInfo.FileName = winpath + @"\system32\notepad.exe";
            process.Start();
            process.WaitForInputIdle();
            {
                Thread.Sleep(200);
                SetParent(process.MainWindowHandle, this.panelClient.Handle);
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }

    private void buttonSendKeys_Click(object sender, EventArgs e)
    {
        if (process == null)
            return;

        // https://social.msdn.microsoft.com/Forums/en-US/eebaa35b-f22c-47b2-9fc9-2a5e876f0b79/problem-with-sendmessage-in-c?forum=csharplanguage
        UInt32 WM_KEYDOWN = 0x0100;
        UInt32 repeatCount = 13;
        UInt32 scanCode = 0x2D;
        UInt32 extended = 0;
        UInt32 context = 0;
        UInt32 previousState = 0;
        UInt32 transition = 0;

        // combine the parameters above according to the bit
        // fields described in the MSDN page for WM_KEYDOWN
        UInt32 lParam = repeatCount
            | (scanCode << 16)
            | (extended << 24)
            | (context << 29)
            | (previousState << 30)
            | (transition << 31);


        IntPtr editHandle = FindWindowEx(process.MainWindowHandle, IntPtr.Zero, "EDIT", null);

        PostMessage(editHandle, WM_KEYDOWN, 'A', lParam);  
    }
    //...
}