As explained in the documentation of the plugin, the “http://localhost:8000/” corresponds to a separate, standalone Web service (i.e. it is not Orthanc) that takes care of granting access to the resources. Obviously, you are not obliged to use 8000, any port different than the TCP ports used by Orthanc is OK. An example written in node.js is provided: https://book.orthanc-server.com/plugins/authorization.html#expected-answer