How to get the OTR number from the freight order item (ITEM_TR)

Hi experts,

for a form, I am starting from the freight order items. In TM, I can see the OTR number at the item of the freight order, but when I look into node ITEM_TR (subnode of TOR  ROOT node), I cannot see the OTR number. How can I get the OTR number from the freight order item? In my case, the relationship between freight unit and OTR is 1:1.

Thank you for a hint. I understand retrieve and retrieve by association, please give me association and field Name.

Thanks so much

Petra