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. */