define file manager drop action when modifier key pressed

If I drag a file from one file manager window and drop it in another, the file gets moved from the source folder to the destination folder. If I hold down the control key while doing this, the file gets copied. If I hold down the Shift key while doing this, the file is also moved. Holding down the Alt key causes the window to be moved instead, so I'm ignoring that case.

I would like to change this behavior so that when the Shift key is held down when the file is dropped, a script of my choosing is running which gets the source path/file and destination path. Alternately, it would work if I could copy the path on the source side from the menu or with the keyboard shortcut key (e.g. Ctrl + C) and then have a menu option / shortcut key that performs the action in the target window.

My main (but not only) use for this will be to do things like create hard/soft links. Another thing I want to do is to create a script that, if the file already exists in the destination directory, checks if the files have identical contents, and if they do, it instead deletes the source file.

I'm using PCManFM on Lubuntu 18.04, but if another file manager is the only way to do it that's fine too.