CN_SETFOCUS - Syntax

The container control sends a WM_CONTROL (in Container Controls) message with the CN_SETFOCUS notification code to its owner whenever the container receives the focus.

param1
     USHORT  id           /*  Container control ID. */
     USHORT  CN_SETFOCUS  /*  Notification code. */

param2
     HWND    hwndCnr      /*  Container control handle. */