Add "Share Page" button to topic toolbar

[mrmansfan]

I would like to add a custom button (JavaScript) to my HTML5 topic toolbar that allows me to share the current page (copy the URL). Does anyone have a working script for this button? I tried a search but apparently, I don't have the language I need to find what I'm looking for. I'd like something similar to what YouTube has, where you can click "share," and it pops up a selected URL that the user can copy/paste into an email.