How to Update customized fields in InventTrans after posting PO


I doing migration from 2009 to 2012 R3.

I have added customized fields related to Amortization in PurhTable form , the data enterd is getting update in Purchline table after PO.

I want the customized field to be updated in InventTrans table also which are added in both purchline and Invent Trans table.

For that purpose in 2009 there is insert method written in PurchFormLetter_Invoice class → Updatenow method. So by using that method data is getting update in InventTrans, but in 2012 R3 there is no update methods. So where should write that code to call the method to update data in InventTrans table.