How to install JScript describe in Copy Selected Filenames

Re this FAQ:

At the end there is a J Script. Should it be a menu, a button, or something else?

I can't figure out how to setup the JScript. I'd be grateful for a clue.

A button (or hotkey). The screenshot above the script shows what things should look like in the button editor.

The second post, "Scripts" section, here has more detail if you need it: