Add a functionality to the 'Purchase Order' page.

Hello,

Now I'm working on 'Purchase Order' page. Within the 'Lines' fasttab there is a field called 'Department Code'. I need to add a functionality to this field. If an user doesn't select any options for this field then the 'Purchase Order' page will not be posted. User have to select an option in this field. How could I implement this? If you help me with the code & where to write then it will be very helpful to me.

Regards,