The container control sends a WM_CONTROL (in Container Controls) message with the CN_DROPNOTIFY notification code to its owner when a pickup set is dropped over the container.
param1
USHORT id /* Container control ID. */
USHORT CN_DROPNOTIFY /* Notification code. */
param2
PCNRLAZYDRAGINFO pCnrLazyDragInfo /* Pointer to the CNRLAZYDRAGINFO structure. */