Pass user ID to core-ajax method GET to retrieve data in Google Cloud storage

I have an app in google engine with a go backend. I'm trying to retrieve a json file that I previously saved in google cloud storage. The backend is based on polymer and javascript. The problem is that the data need to be retrieved using the user-id through a core-ajax call.

Here is the javascript code I'm using at the moment:

loadTrials : function loadTrials() {
            var _this = this,
                load = this.shadowRoot.querySelector('#load-trial');

            load.url="http://url/loadTrials";
            load.go();
            load.addEventListener('core-response', function(ev) {
                var json = ev.detail.response;
                _this.trialData = JSON.parse(json)['trial-data'];
                console.log(JSON.parse(json)['trial-data'])
            }, false);
        }

The HTML

<core-ajax id="load-trial"  method="GET"></core-ajax>

And the backend in go(just the relevant part):

func handleloadTrials(w http.ResponseWriter, r *http.Request) {
	
	defer r.Body.Close()

	//retrieve trials
    http.ServeFile(w, r, JSON file related to user id)

}

I know this code is wrong, but I'm completely stuck and I don't know how to pass the user ID to the GET method, and then use the ID to retrieve the specific file on cloud storage (it's a json file) and then pass this Json file to the http.ServeFile method in Go.

Any help?
Thanks a lot.

<core-ajax url="{{url}}" handleAs="json" params='{"userID": "{{userID}}"}'></core-ajax>

The parameter should be on JSON format.

I partially solved in this way

Javascript:

loadTrials : function loadTrials() {
            var _this = this,
                load = this.shadowRoot.querySelector('#load-trial');
            //pass userID to core-ajax
            _this.userID = _this.app.userSession.user;
            load.url="http://url/loadTrials";
            load.go();
            load.addEventListener('core-response', function(ev) {
                var json = ev.detail.response;
                _this.trialData = JSON.parse(json)['trial-data'];
                console.log(JSON.parse(json)['trial-data'])
            }, false);
        }

HTML:

<core-ajax id="load-trial" params="{{userID}}" method="GET"></core-ajax>

Go:

type trialStruct []struct {
	Trial struct {
		Index           int    `json:"index"`
		Word            string `json:"word"`
		WordDisplayTime int    `json:"wordDisplayTime"`
		AnswerMaxTime   int    `json:"answerMaxTime"`
		FixationTime    int    `json:"fixationTime"`
		Train           bool   `json:"train"`
		Test            bool   `json:"test"`
		Grammatical     bool   `json:"grammatical"`
		Grammar         string `json:"grammar"`
		keyboard        bool   `json:"keyboard"`
	}
	Metadata struct {
	}
}

func (d *saveData) readFile(fileName string) trialStruct {
	trialName := fileName + "/trials"
	rc, err := storage.NewReader(d.ctx, bucket, trialName)
	if err != nil {
		d.errorf("readFile: unable to open file from bucket %q, file %q: %v", bucket, trialName, err)

	}
	defer rc.Close()
	slurp, err := ioutil.ReadAll(rc)
	if err != nil {
		d.errorf("readFile: unable to read data from bucket %q, file %q: %v", bucket, fileName, err)
	}
	var trialJson trialStruct
	err1 := json.Unmarshal(slurp, &trialJson)
	if err != nil {
		log.Printf("error:", err1)
	}
	return trialJson
}

But now I have the Encoded Json file which I'm not sure how to send back to the javascript, any help? Thanks