Hi Gideon,
That's a good question. Whilst I told you to just supply the server URL (i.e. http://gis:6080) when doing an Add Data from Server, the URL you need to give for "Add feature service URL to AXF" is the deeper REST API for your feature service.
It would look something like: https://external/arcgis/rest/services/GIS_MAP/FeatureServer
When you expose a Feature Service externally, it is up to you to choose the URLs and how they map to your internal server. e.g. you may say that https://external/arcgis/ maps to http://gis:6080/arcgis/ - it's really up to you (website administrator) to choose how much or how little of your Feature Service you want to expose.
Stephen