Example

//...
[Parameter]
public DataSeries Source { get; set; }

[Parameter(DefaultValue = 15)]
public int periodfast { get; set; }

[Parameter(DefaultValue = 25)]
public int periodslow { get; set; }

private SimpleMovingAverage smafast;
private SimpleMovingAverage smaslow;

protected override void OnStart()
{
    smafast = Indicators.SimpleMovingAverage(Source, periodfast);
    smaslow = Indicators.SimpleMovingAverage(Source, periodslow);
}
protected override void OnTick()//This could be OnBar() also.
{
    {
        if (smafast.Result.LastValue > smaslow.Result.LastValue)
        {
            Trade.CreateSellLimitOrder(Symbol,10000,Symbol.Bid-Symbol.PipSize,null,null,null)
        }
    }
}
//...