Unity embed EXE in game window

using UnityEngine;
using System.Collections;

public class LoadChild : MonoBehaviour {

    // Update is called once per frame
    void Update () {
        if(Input.GetKeyDown(KeyCode.D))
        {
            int thisHandle = GetWindowHandle().ToInt32();
            System.Diagnostics.Process.Start("child.exe","-parentHWND "+thisHandle);
        }
    }

    [System.Runtime.InteropServices.DllImport("user32.dll")]
    private static extern System.IntPtr GetActiveWindow();

    public System.IntPtr GetWindowHandle()
    {
        return GetActiveWindow();
    }
}