BADI trip_web_check

I am working on an enhacment in travel mangement area. The user wud create a travel expense receipt in portal and when they try to save the receipt I shud trigger this BADI . In badi we need to check for certain expense types if the total amount has been itemised or not. if not pop up and error message on portal side saying its needs to be itemised. The BADI which was suggested was BADI_TRIP_CHECK. How do I go about doing this ?