Perhaps the Event URL field is new to version 3? I did a bunch of testing with version 2 and the free version and the community add on in a brand new install and did not see that field in the front end or back end. I upgraded to the latest version of each plugin and there it is.
I am having the same issue with the curious addition that I don’t see the URL field in the back end. I am using the Community Events add on with a custom child theme of Genesis. The gist snippet did not put the event URL field on the form for me. Thanks.