Adding an "Add to favorites" or "bookmark this page" link/button to a landing page

I need to add an “add to favorites” link to my landing page. I tried using the custom HTML and javascript pieces to add the code I found at this page: https://rudrastyh.com/javascript/favorite-button.html

I’m wondering if I missed something or if that’s just not going to work in Unbounce at all. In my editor, I have this error message:

Thanks for any suggestions!