The container control sends a WM_CONTROL (in Container Controls) message with the CN_INITDRAG notification code to its owner when the drag button is pressed and the pointer is moved while the pointer is over the container control.

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

param2
     PCNRDRAGINIT  pCnrDragInit  /*  Pointer to the CNRDRAGINIT structure. */