Is it expected that each business creates their own? I tried putting in a generic web site and it did not work.
if I remember right I've seen this error before. Not exactly sure what the cause was, but please try 2 things. Is the MDM Authority correctly set to Intune Standalone for example? You should have been asked during initial setup of your tenant. Second does the user you are enrolling has an EMS license assigned? If I remember right this was my error when I've seen this message.
Regarding your question about the URL. You should use the MS predefined default one. I do not know any feature where you have to change the default one.