Capture a hidden dialog c#

capture.cs

 private async void button1_Click(object sender, EventArgs e)
{
    await Task.Delay(1000);
    await Task.Run(() =>
    {
        var fileName = @"C:\Users\dan\Desktop\Folder\bitmap.png";
        var subwindow = new Slave();
        subwindow.ShowInTaskbar = false;
        subwindow.Show();
        subwindow.DesktopLocation = new Point(int.MinValue, int.MinValue);
        var drawing = new Bitmap(subwindow.Height, subwindow.Width);
        subwindow.DrawToBitmap(drawing, new Rectangle(new Point(0, 0), new Size(subwindow.Width, subwindow.Height)));
        drawing.Save(fileName);
        // Process.Start(fileName);
        subwindow.Close();
    });
}