You can elect to attach an FX Order in cases where you are buying a contract in a currency other than your base, and want to convert base currency to the currency of the contract to cover the cost of the trade.
To attach an FX Order
Create a buy order on a contract.
Right-click in the order line, and select Attach, then select FX Order.
Click the Attributes box in the Status field to show the hedge attributes on the inline editor. The appropriate currency pair is displayed.
Hold your mouse over the FX icon to see the calculated estimated quantity of the hedging contract.
Transmit the parent order. The attached fx order will sell your base currency and buy a quantity of the needed currency to cover the parent trade.