Using the Httpcaller i can successfully make a rest api call to retrieve a token for an ArcGIS Portal Featureservice. A proxy server is set up but is not used for this url so the call bypasses the proxy.
However, I can only set up and use an ArcGIS Portal Feature Service connection if there is no proxy set up at all. If the proxy is enabled, the connection tries to use the proxy even though the base url is on the exception list.