Hi,
Open Orders in Linnworks.net currently supports displaying the Reference number field in the General Info column and will shortly also be allowing for the display of the External Reference number. These are added by editing the view, documentation on editing views can be found here.
https://www.linnworks.com/support/order-book/open-order/using-the-open-order-screen/open-order-working-with-views
https://www.linnworks.com/support/order-book/open-order/using-the-open-order-screen/open-order-working-with-views/working-with-views-modifying#columnData

Mark Aldous
Head of Implementation
Linn Systems Ltd