//**********WinAPI функции*********

        [DllImport("user32.dll", EntryPoint = "FindWindow")] 
        private static extern IntPtr FindWindow(string _ClassName, string _WindowName); 
 
        [DllImport("user32.dll", EntryPoint = "FindWindowEx")] 
        private static extern IntPtr FindWindowEx(IntPtr hwndPrnt, IntPtr hwndChildAfter, string _ClassName, string _WindowName); 

        [DllImport("user32.dll", CharSet = CharSet.Auto)] 
        public static extern int SendMessage(IntPtr hwnd, int wMsg, IntPtr wParam, IntPtr lParam);

        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
        public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);

        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
        public static extern int DestroyWindow(IntPtr hWnd);

//*********/WinAPI функции*********



**********WinAPI константы*********
        const int BM_SETSTATE = 243; 
        const int WM_LBUTTONDOWN = 513; 
        const int WM_LBUTTONUP = 514; 
        const int WM_KEYDOWN = 256; 
        const int WM_CHAR = 258; 
        const int WM_KEYUP = 257; 
        const int WM_SETFOCUS = 7; 
        const int WM_SYSCOMMAND = 274; 
        const int SC_MINIMIZE = 32;
        const int WM_CLOSE = 0x0010;
//*********/WinAPI константы*********

...
...
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private void SentMes_Click(object sender, EventArgs e)
{

            IntPtr hWnd = FindWindow(null, comboBox1.SelectedItem.ToString()); 
            IntPtr t1;
            if (hWnd == IntPtr.Zero) 
            { 
                MessageBox(new IntPtr(0), "Not found main", "Error", 0);
                return; 
            }
            

            retValue = SendMessage(hWnd, WM_SETFOCUS, nul, nul);
            retValue = SendMessage(hWnd, WM_KEYDOWN, t1, nul);
            retValue = SendMessage(hWnd, WM_KEYUP, t1, nul);
}