Issue with async Method. EIther i cant convert the type or button.click wont work

Question

this is my code:

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            SetContentView(Resource.Layout.Main);

            //Initialize buttons:

            Button StartButton = FindViewById<Button>(Resource.Id.startbutton);
            TextView txtTestGps = FindViewById<TextView>(Resource.Id.GpsTest);

            ShowGpsCoordinates(StartButton, txtTestGps);
        }

        private async void ShowGpsCoordinates(Button StartButton, TextView txtTestGps) 
        {
            await StartButton.Click += (sender, e) =>
            {
                double xy = await GiveGpsLocation();
                txtTestGps.Text = xy; 

            };
        }


        private async Task<double> GiveGpsLocation()
        {
            double DoubleWithCoordinates = 0.0;

            var locator = CrossGeolocator.Current;
            locator.DesiredAccuracy = 50;

            var position =  await locator.GetPositionAsync(10000);

            // Console.WriteLine("Position Status: {0}", position.Timestamp);
            // Console.WriteLine("Position Latitude: {0}", position.Latitude);
            // Console.WriteLine("Position Longitude: {0}", position.Longitude);

            DoubleWithCoordinates = position.Latitude; 

            return DoubleWithCoordinates;  
        }




    }
}

but the part with the startbutton.click wont work. I made everything async so that i can convert task to doubles. But now i get: "The Event View.Click can only appear on the left hand side of += or -="

But, as far as i know, it is on the left hand side. Anyway, I think the problem is the async method, because before everything worked just fine. Only that there was no way to convert Task to x. Thats why I made the methods by async. Can any of you guys help me with my problem?

Regards :)