Goods receipt importing purchase order

Hi,

I have an addon that has to fill some udf's when a Itemcode is entered in the detail section. This works relativaly ok. But what is the best way to force the filling the udf's when the lines are filled in through the selection of a purchase order.

For the moment I have a routine that fill's all the udf is the screen gets activated and the button purchase order was clicked. This works if the user doesn't click on the good receipt while the wizard for importing purchase order is open.