Purchase order creation updation

Hi all ,

Have a query on ax 2012 …I created a method on purchtable to bring tax group on purchtable and purchline table from my own table.

but where should I introduce my method in purchtable form so that everytime a new manual PO is made from anywhere like MFG , SO , SVC … It should go in my method to pick the tax group…

Can anybody let me know the answer…