Adding multiple URLs to an entry means that autofill will across multiple different domains.
When editing an entry, there's only space for a single URL in the URL field. To add additional URLs:
1. Edit an entry
2. Create a new custom field
3. Name the "URL" followed by a number; for instance "URL2"
4. Enter the URL itself into the Value field
5. Save the entry
Now when you visit the new URL in your browser, you will be able to autofill your login credentials.