You can create whatever id you want. So, you could put in "worldswithoutend:1234" and calibre will store and display it. But, to translate that to a URL, you need a Metadata Source plugin. With that, calibre can search for the book on the site and retrieve the metadata and cover. Creating a metadata source plugin isn't particularly hard, especially if the site is well designed. You need some Python and HTML knowledge. If you are interested in creating one, have a look at any of the metadata source plugins listed in the
plugin index. They can be used as a starting point.
The alternative is to use a URL identifier. With this, you use "url:https://worldswithoutend.com/novel.asp?id=2858". When you select the book, the details panel will show the "worldswithoutend.com" and clicking it will open the page.