The container control sends a WM_CONTROL (in Container Controls) message with the CN_DROPHELP notification code to its owner when the container receives a DM_DROPHELP message.

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

param2
     PCNRDRAGINFO  pCnrDragInfo  /*  Pointer to a CNRDRAGINFO structure. */