Hi,
Please check the note 481600 which gives details bout the import tab in po.
You can check the incompleteness procedure in customizing, which can trigger message while saving the po. SPRO Pat: Materials Management -> Purchasing -> Foreign Trade/Customs -> Control Foreign Trade Data in MM and SD Documents -> Incompleteness Schemas for Foreign Trade Data
Here, you can add the field to incompleteness procedure. Select the required Schema & click on document fields, this gives the list of fields for which you can customize the controls for field selection & incompleteness.
Hope its helpful.
Regards,
AKPT