Extensions.cs

public static void TryRequestWhenInUseAuthorization(this CLLocationManager locationManager)
{
  if (locationManager.RespondsToSelector(new Selector("requestWhenInUseAuthorization")))
    locationManager.RequestWhenInUseAuthorization();
}

public static void TryRequestAlwaysAuthorization(this CLLocationManager locationManager)
{
  if (locationManager.RespondsToSelector(new Selector("requestAlwaysAuthorization")))
    locationManager.RequestAlwaysAuthorization();
}

info.plist

<key>NSLocationWhenInUseUsageDescription</key>
<string>Message to the user</string>

or

<key>NSLocationAlwaysUsageDescription</key>
<string>Message to the user</string>

iOS8Check.cs

if (UIDevice.CurrentDevice.CheckSystemVersion(8, 0))
{
  _locationManager = new CLLocationManager();
  _locationManager.AuthorizationChanged += LocationManagerOnAuthorizationChanged;
  _locationManager.TryRequestWhenInUseAuthorization();
}
else
{
  _mapView.ShowsUserLocation = ViewModel.LocationPermission;
}

MapViewController.cs

var locationManager = new CLLocationManager();
locationManager.AuthorizationChanged += LocationManagerOnAuthorizationChanged;
locationManager.TryRequestWhenInUseAuthorization();

// create your map etc.

private void LocationManagerOnAuthorizationChanged(object sender, CLAuthorizationChangedEventArgs args)
{
  if (args.Status == CLAuthorizationStatus.AuthorizedWhenInUse ||
      args.Status == CLAuthorizationStatus.AuthorizedAlways)
    _mapView.ShowsUserLocation = true;
  else
    _mapView.ShowsUserLocation = false;
}