Attaching Windows Form To Unity3D made game

Question

It works on other winform programs, but not in the game itself. I'm trying to make an extra menu or "mod menu" but it's invisible.

The button for message box to test if winform works. When I click "nowhere" in up left corner, it shows me the "hello" in the message box.

I tried editing the code but unable to find the problem. What should I do?

namespace myapp
{

public partial class Form1 : Form
{
    [System.ComponentModel.Browsable(false)]
    public bool AllowTransparency { get; set; }

    [System.ComponentModel.Browsable(false)]
    public bool AllowTransparencykey { get; set; }

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

    [DllImport("user32.dll")]
    public static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);

    public static int GWL_STYLE = -16;
    public static int WS_CHILD = 0x40000000;

    public Form1()
    {
        InitializeComponent();

    }

    private void Form1_Load(object sender, EventArgs e)
    {

    }

    private void button1_Click(object sender, EventArgs e)
    {
        Process hostProcess = Process.GetProcessesByName("game").FirstOrDefault();
        if (hostProcess != null)
        {
            Hide();
            FormBorderStyle = FormBorderStyle.None;
            SetBounds(0, 0, 0, 0, BoundsSpecified.Location);

            IntPtr hostHandle = hostProcess.MainWindowHandle;
            IntPtr guestHandle = this.Handle;
            SetWindowLong(guestHandle, GWL_STYLE, GetWindowLong(guestHandle, GWL_STYLE) | WS_CHILD);
            SetParent(guestHandle, hostHandle);
            Show();
            FormBorderStyle = FormBorderStyle.None;
            SetBounds(0, 0, 0, 0, BoundsSpecified.Location);
        }

    }

    private void button2_Click(object sender, EventArgs e)
    {
        MessageBox.Show("Hello");
    }
}
}