A SimpleDataField with WebRequest

I'm a bloody beginner and I'd like to return the data from a web request:

import Toybox.Activity;

import Toybox.Lang;

import Toybox.Time;

import Toybox.WatchUi;

class AareTempView extends WatchUi.SimpleDataField {

    // Set the label of the data field here.

    function initialize() {

        SimpleDataField.initialize();

        label = "°C";

    }

    // The given info object contains all the current workout

    // information. Calculate a value and return it in this method.

    // Note that compute() and onUpdate() are asynchronous, and there is no

    // guarantee that compute() will be called before onUpdate().

    function compute(info as Activity.Info) as Numeric or Duration or String or Null {

        // See Activity.Info in the documentation for available information.

        return „water temperature from url https://aareguru.existenz.ch/v2018/current?city=thun&version=1.0.42&values=aare.temperature“; 

    }

}

I've heard that it's only possible with a background service, but I don't how to combine the code above with the following samples (JSON, backgrounding):

https://developer.garmin.com/connect-iq/core-topics/https/ 

https://developer.garmin.com/connect-iq/core-topics/backgrounding/ 

Is anyone interested in helping a non information scientist?

Watch: Instinct 2 Solar Surf Edition