You can use the Crossplatform Geolocator plugin developed by jamesmontemagno.
Get coordinates as:
var locator = CrossGeolocator.Current;
locator.DesiredAccuracy = 50;
var position = await locator.GetPositionAsync (timeoutMilliseconds: 10000);
Console.WriteLine ("Position Latitude: {0}", position.Latitude);
Console.WriteLine ("Position Longitude: {0}", position.Longitude);
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