The container control sends the WM_CONTROL (in Container Controls) message with the CN_BEGINEDIT notification code to its owner whenever container text is about to be edited.

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

param2
     PCNREDITDATA  pCnrEditData  /*  Pointer to the CNREDITDATA structure. */