Is it Gluu CE, or Gluu CN/Docker Edition? Also, you need to make sure this url you submit on that page is accessible by oxTrust (web UI). So it must be resolvable from inside container (try to ping for its FQDN from inside there), and if it's HTTPS uri, then oxTrust needs to trust site's certificate.
Please also tail (`tail -F`) web UI's log at `/opt/gluu/jetty/identity/logs/oxtrust.log`, then try to update the URI again. Post here the errors that will be logged there, if any.