To hedge all positions in the Account:

        [Parameter(DefaultValue = 100000)]
        public int Volume { get; set; }

        protected override void OnStart()
        {
            foreach (var position in Account.Positions)
            {
                if (position.TradeType == TradeType.Buy)
                {
                    Trade.CreateSellMarketOrder(Symbol, Volume);
                    //break; // optional break to hedge only one position
                }
                else
                {
                    Trade.CreateBuyMarketOrder(Symbol, Volume);
                    //break; // optional break to hedge only one position
                }
            }
        }

OnPositionOpened is triggered when the pending order is filled. Positions do represent market orders. Do you need help with the code to do hedging?