Hi Dj,
You want to display item number in the AET field means then redefine getter & setter methods of the field and retrieve the item number through GET_RELATED_ENTITIES method by passing the relation name as 'ExtBTRefItemRel'.
If you using any BAdI to auto populate the item number please check this note 1660483 - Error in Complaint creation with reference to Invoice
Hope this help you.
Regards,
Gopalakrishnan S.