I was fooling around hours ago (still), and started to search a chapter (where in the documentation) about How do I add a Drop into the Application Icon ?
At last, I found (comparing with an old project where it works) how to do it, but I usually love to read how to do in the documentation.
Any hint on where to find that in the documentation ?
Thank you Antonio.
It seems that my medicine is doing bad things to my brain. I will talk about that with my Doctor tomorrow afternoon.
I was searching App, Application, DragItem, then blank (or black) and leave the search 
BTW: leaving a document on an application icon is a different beast: we have to set more stuff. Save code, but some things have to be done to enable the drag.
Any link for that one ?
You have to define the fileTypes that you accept. Then select them in the macOS settings (Choose button near FileTypes)
Thank you Antonio.
Now, after many hours of search, I know how to do that.
But what I miss (we all miss ?) is where the description is in the documentation.
Usual suspects do not answer so I assume nobody knows. I will wrote a pdf and explain (later in the afternoon).
I cannot believe this feature does not have a description / no one ask LOUD for it. (I found a thread from last year or so on this subject that ends without answer.)