Missing parameter for iMA function

I have the following line of code in my indicator:

ExtMapBuffer1[pos]= iMA(NULL,0,FasterMA,0,MODE_LWMA,PRICE_CLOSE,pos);

Instead of PRICE_CLOSE I want to use the previous indicators data. If you add an MA onto your chart you will see the "Apply to" listbox which contains the option of previous indicators data. This is not an option according to the help files in MT4. How do you do this?

Thanks,

Jason