Amazon UK requires IEN's entered in the shipment/tracking line. I called FedEx many times trying to get the IEN but they could never help. Finally, I found it in their documentation. Here is how to get it:

Log into your FedEx account, go to support/ billing & invoicing. click on the invoice for the shipment, then click on the tracking number, then click on "custom document" once it's generated, download it and open it. On the 3rd page-Import Data Summary, top left corner is the entry number which is the "IEN"