SendCommand

        [DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
        static extern IntPtr FindWindowByCaption(int ZeroOnly, string lpWindowName);

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, int wParam, IntPtr lParam);

        const int WM_KEYDOWN = 0x100;
        //const int WM_KEYUP = 0x101;
        const int L_KEY = 0x4C;

        private void button1_Click(object sender, EventArgs e)
        {
            IntPtr ip = FindWindowByCaption(0, "Untitled - Notepad");

            SendMessage(ip, WM_KEYDOWN, L_KEY, IntPtr.Zero);
            //SendMessage(ip, WM_KEYUP, L_KEY, IntPtr.Zero);
        }