// Alexander Klimov
// http://developer.alexanderklimov.ru/
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern Int32 SendMessage(IntPtr hWnd, int msg,
int wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);
private const int EM_SETCUEBANNER = 0x1501;
SendMessage(textBox1.Handle, EM_SETCUEBANNER, 0, "Type your question");