How do i place an order?

I havethis but it is not working: 
                OrderConfiguration orderConfiguration = new OrderConfiguration() { Market_Market_Loc = new MarketMarketLoc(){Base_Size = "20.00"} };
                var ord = await _coinbaseAtClient.OrdersService.CreateOrderAsync(Guid.NewGuid().ToString(),
                    "BTC-USD", "sell", orderConfiguration);