An Esri ArcGIS Portal Feature Service writer is supported for 2016 though I've never got it to work. The Beta testing for this writer, Safe Support helped me test it, but we never got it working successfully. @danpaoly,for what you are asking, "to load an AGS feature layer on Portal", can you please clarify what exactly you want to do? If you will do this manually in portal how would you do it? If I understood your question correctly, the feature layer is added as an item in Portal to be discoverable in the Portal by users. This is different from an AGS feature layer that is "loaded" or added in Portal. The writer supported by FME

creates and hosts the feature data in its managed data and then publishes the feature layer into the Portal. To load a service URL directly in Portal I normally just use sign in and share features to Portal in ArcGIS Server Manager or ArcMap and the service URL is then added in to the Portal. In ArcPY, I'm not sure if an add item to Portal is already supported, if it is, then you could use either a Python Caller transformer or the shutdown python script on your workspace. Hope that helps.

I just received an email for my Portal support case. The writer that works with various authentication methods is now available in Beta. I tested it and was able to create the data and publish the feature layer. There's a bug if you want to use an existing folder and I already sent that feedback. For our Portal installation, I used NTLM. The web adaptor URL that worked for our portal is https://portalname.companydomian.com/portal. If I do not include /portal I get an error, so please be aware. Based onthe Installed shortcut this beta is to be releaseas as FME Desktop 2016.1 (Build 16478) (32-bit). Here's the beta download link: http://www.safe.com/support/support-resources/fme-downloads/beta/