Why would you need to program a mouse button for a function that you can already do through the right-click contextual menu?
I see that you did mention the "mouse popup menu", but right-click, with the contextual menu is your go-to for that kind of operation.
When you select text, right click, and choose copy. Click on your desired destination location, and right-click again, choosing paste. There are other choices available through the right-click contextual menu, rather than limiting a button on your mouse to a single function. That might have been a good choice when the operating systems were relatively limiting already, years ago. But, make use of what the Finder provides for you.