Keyword 'this' is not valid in a static property, static method, or static field initializer

public partial class ThisAddIn
{
    private const int WH_KEYBOARD_LL = 13;
    private const int WM_KEYDOWN = 0x0100;

    private static IntPtr hookId = IntPtr.Zero;
    private delegate IntPtr HookProcedure(int nCode, IntPtr wParam, IntPtr lParam);
    private static HookProcedure procedure = HookCallback;

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr GetModuleHandle(string lpModuleName);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool UnhookWindowsHookEx(IntPtr hhk);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr SetWindowsHookEx(int idHook, HookProcedure lpfn, IntPtr hMod, uint dwThreadId);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
        hookId = SetHook(procedure);

    }

    private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
    {
        UnhookWindowsHookEx(hookId);
    }

    private static IntPtr SetHook(HookProcedure procedure)
    {
        using (Process process = Process.GetCurrentProcess())
        using (ProcessModule module = process.MainModule)
            return SetWindowsHookEx(WH_KEYBOARD_LL, procedure, GetModuleHandle(module.ModuleName), 0);
    }

    private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
        {
            int pointerCode = Marshal.ReadInt32(lParam);
            string pressedKey = ((Keys)pointerCode).ToString();

            //Do some sort of processing on key press
            var thread = new Thread(() =>
            {   MessageBox.Show(pressedKey);
                if (pressedKey.Equals("Space") || pressedKey.Equals("Tab"))
                    {
                        Word.Range rng = this.Application.ActiveDocument.Words.Last;
                        rng.Select();
                        rng.Copy();
                        String input = Clipboard.GetText(TextDataFormat.Text);
                    }
            });
            thread.Start();

        }
        return CallNextHookEx(hookId, nCode, wParam, lParam);
    }

    private void InternalStartup()
    {
        this.Startup += new System.EventHandler(ThisAddIn_Startup);
        this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
    }
}