XmGetDestination man page
XmGetDestination — A function that returns the widget ID of the widget to be used as the current destination for quick paste and certain clipboard operations
#include <Xm/Xm.h> Widget XmGetDestination( Display *display);
XmGetDestination returns the widget that is the current destination on the specified display. The destination is generally the last editable widget on which a select, edit, insert, or paste operation was performed and is the destination for quick paste and certain clipboard functions. The destination is NULL until a keyboard or mouse operation has been done on an editable widget. Refer to the Motif Programmer's Guide for a description of keyboard focus.
Specifies the display whose destination widget is to be queried
Returns the widget ID for the current destination or NULL if there is no current destination.