Release Procedure based on Account assignment category,Customer Exit

Hi,

I am trying to set release strategy based on Account assignment category for Purchase Order.

As KNTTP field is not available in communication structure CEKKO, I have appended the structure.

Now I have created characterestics, class and defined release procedure.

When I used "Check Release Strategies", I find a warning message "You must maintain a customer exit for characteristic

PO_ACC_CAT"

When I create Purchase Order, system is not picking up the strategy that I have defined for Asset Purchase Orders. Other strategies only are working fine.

Could anyone suggest. I need the Code that we need to maintain for the Customer Exit.

Your help would be highly appreciated.

Thanks.