Changing unauthenticated property from true to false solved 403 Forbidden but raises new Json exception.
Changing GetMultiple method to GetSingle solved Json exception.
//Bug in GetManagedVenues() Method
List<FourSquareEntityItems<Venue>> venueGroups = GetMultiple<FourSquareEntityItems<Venue>>("/venues/managed", parameters, false).response["venues"];
//Solved replacing that line with this
FourSquareEntityItems<Venue> venues = GetSingle<FourSquareEntityItems<Venue>>("/venues/managed", parameters, false).response["venues"];