Okay, this seems really sad answering my own question, but just in case anyone else is having the same problem, I have found the solution:
In the 'advanced button' 'properties' box, do the following:
- In Button Type select OnClick
- In OnClick Action select Execute Java Script function
- In OnClick Value type:
window.external.AddFavorite(location.href, document.title);
This will create a button that, when clicked, will display the 'add to favorites' pop-up box with the page title and URL already filled in.