How Do I Get Screenshot Without My Winform On It?

/*
    Referenced assemblies:
    1) System;
    2) System.Windows.Forms;
    3) System.Drawing;
    4) Nothing else.
*/
namespace HideAndCopyForm {
    using System;
    using System.Windows.Forms;
    using System.Drawing;

    public class MainForm : Form {

        public MainForm() {
            Button btn = new Button();
            btn.Text = "&Test";
            btn.Parent = this;
            btn.Click += (sender, eventArgs) => {
                this.Opacity = 0;
                System.Windows.Forms.Clipboard.SetImage(ScreenShot());
                this.Opacity = 1;
            }; //btn.Click
        } //MainForm

        Image ScreenShot() {
            var screenSize = Screen.PrimaryScreen.Bounds.Size;
            var image = new Bitmap(screenSize.Width, screenSize.Height);
            using (var g = Graphics.FromImage(image)) {
                g.CopyFromScreen(Point.Empty, Point.Empty, screenSize);
            }
            return image;
        } //ScreenShot

        [STAThread]
        static void Main() {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
        } //Main

    } //class MainForm

} //namespace HideAndCopyForm