[amibroker] using custom N-tick bars in backtest

[onelkm]

I want to evaluate custom n-tick bars in a back test. I set up the chart in 
Preferences>Intraday>custom N-tick and I also set the tick size in symbol 
information. Everything charts just fine. Now I want to run a back test, but in 
AA>Settings>Periodicity there is no option to choose any N-tick periods. Is 
there a way to do this?
Thanks for any help
Larry