How to implement the global hook?

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C#

Hi guys... im trying to work on global hook and i tried browsing for some example and i tackled on one of this site that posted this code

 private delegate int LowLevelKeyboardProcDelegate(int nCode, int
wParam, ref KBDLLHOOKSTRUCT lParam);
        [DllImport("user32.dll", EntryPoint = "SetWindowsHookExA",
       CharSet = CharSet.Ansi)]
        private static extern int SetWindowsHookEx(int idHook,
        LowLevelKeyboardProcDelegate lpfn, int hMod, int dwThreadId);
        [DllImport("user32.dll", EntryPoint = "UnHookWindowsHookEx",
       CharSet = CharSet.Ansi)]
        private static extern int UnHookWindowsEx(int hHook);
        [DllImport("user32.dll", EntryPoint = "CallNextHookEx",
       CharSet = CharSet.Ansi)]
        private static extern int CallNextHookEx(int hHook, int nCode, int
        wParam, ref KBDLLHOOKSTRUCT lParam);
        const int WH_KEYBOARD_LL = 13;
        public struct KBDLLHOOKSTRUCT
        {
            public int vkCode;
            int scanCode;
            public int flags;
            int time;
            int dwExtraInfo;
        }
        private int intLLKey;
        private KBDLLHOOKSTRUCT lParam;
        private int LowLevelKeyboardProc(int nCode, int wParam, ref
KBDLLHOOKSTRUCT lParam)
        {
            bool blnEat = false;
            switch (wParam)
            {
                case 256:
                case 257:
                case 260:
                case 261:
                    //Alt+Tab, Alt+Esc, Ctrl+Esc, Windows Key
                    if (((lParam.vkCode == 9) && (lParam.flags == 32)) ||
                    ((lParam.vkCode == 27) && (lParam.flags == 32)) || ((lParam.vkCode ==
                    27) && (lParam.flags == 0)) || ((lParam.vkCode == 91) && (lParam.flags
                    == 1)) || ((lParam.vkCode == 92) && (lParam.flags == 1)) || ((true) &&
                    (lParam.flags == 32)))
                    {
                        blnEat = true;
                    }
                    break;
            }
            if (blnEat)
                return 1;
            else return CallNextHookEx(0, nCode, wParam, ref lParam);
        }
        public void KeyboardHook(object sender, EventArgs e)
{
intLLKey = SetWindowsHookEx(WH_KEYBOARD_LL, new LowLevelKeyboardProcDelegate(LowLevelKeyboardProc) ,
System.Runtime.InteropServices.Marshal.GetHINSTANCE(System.Reflection.Assembly.GetExecutingAssembly().GetModules()[0]).ToInt32(),0);
if(intLLKey == 0){
MessageBox.Show("alt tab was not pressed");
}else{
MessageBox.Show("alt tab waspressed");

}

        }

now i want this code to check or prompt a message box that will say that the key alt+tab was pressed? how do i do this? and where should i placed it? is it during the form load?
i tried placing it in form load but it only prompts once but when i pressed the alt+tab it does not do anything anymore... i placed the it in form load like this

private void Form1_Load(object sender, EventArgs e)
    {
        KeyboardHook(sender, e);
    }

i have no idea how to invoke or call and implement the global hook... please help me with this