The server URL it asks for is the same as the URL for the home page on the server you register on. The main site's home page is
http://www.yetanothersig.com, from where anyone can register for an account.
If you download the server software and install it to another location, http://somedomain.com/yass/and register there, then that would be the URL. Also, if the path is in a folder like in this example, then the trailing slash is needed. eg:
http://somedomain.com/yass
won't work
http://somedomain.com/yass/
works
http://subdomain.domain.com/folder/folder2/yass/
works
to sum it up, http://www.yetanothersig.com is the URL if you register an account there.