Custom Function Help

Custom Function Help

Basically what I am try to do is update the price list automatically. On invoice creation I want it to look to the custom field and if its value is one then set the price list to {1} 

I have a custom field in contacts Label: Price Band, Value: 0 - 6

but the bit im struggling with is I can't find the correct command to get the price list map in the invoice 
its not listed here  https://www.zoho.com/invoice/help/settings/automation/custom-functions.html