I guess he means, for example, selecting some text in another text editor, dragging and droping it on the memo.
AFAICT there is no event to capture it; you'll probably have to intercept whatever message is sent when that happens, which I don't remember ATM which it is (and probably depends on the paltform/widgetset), sorry
I've never had to use something like this, the usual auto-pasting of the text in the memo works well for me, but I can see where it could come in handy, to differentiate between normal editing actions and a text-drop before an OnChange event.
I had a goo look at TMemo's events, and the is no OnDrop. Do you mean OnDragDrop?
That won't work in this context; all the drag-drop related events work only for dragging a control/object and dropping it over another, like one does e.g. in the form designer.