Getting data from an invoke operation

Does somebody have a working example of using an invoke operation in a mobile service, and using it in the mobile app builder?

So I this is my mobile service:

@openapi.openedge.export(type="REST", useReturnValue="false", writeDataSetBeforeImage="false").

@progress.service.resourceMapping(type="REST", operation="invoke", URI="/AuthenticateUser", alias="", mediaType="application/json").

PROCEDURE AuthenticateUser:

DEFINE INPUT  PARAMETER username AS CHARACTER   NO-UNDO.

DEFINE OUTPUT PARAMETER userObj  AS DECIMAL     NO-UNDO INITIAL 0.

...

1) When I test the service with the Postman rest client, and I don't give any parameter (empty http request body), I get the correct response: 

{

    "response": {

        "userObj": 0

    }

}

When I give a request parameter username, I keep getting "415 unsupported media type" , even when i put the request in any json format that I try.

2) When I import the service in the mobile app builder, and i create the service and the edit mapping: then I get the request parameter "username" , but on the response tab everyting is empy, there are no parameters to chose from. So he doesn't recognize the outputparameter userObj of my Progress procedure.

When I test the service in my browser, I was able to intercept the communication, and the browser effectively sends this JSON as request for an empty username:

{"request":{"username":""}}

And gets this response:

{

    "response": {

        "userObj": 0

    }

}

I tried to put the same request JSON format in the Postman rest client, but I keep getting "415 unsupported media type" there.

Also, when I manually create a parameter response-userObj in the response tab in the Mobile App builder, this doesn't receive the value from the response.