New features and order types are being added to IBKR Mobile with each new release. At this time, however, you cannot attach an FX Hedge order to an order created in IBKR Mobile.
As a workaround, if you have a Margin account you can check the Market Value section of your Account window post trade to see if the trade created a currency loan. If so, youcan then create a new FX Conversion order to close the negative currency balance resulting from the trade.