Make a copy of the linked template and fill it out accordingly.
If you need this Cross Reference Number to act as a barcode then you will need to enter "YES" into the 4th column per the template.
Ensure you select the accurate Barcode Type:
UPC barcode must be 12 digits long
EAN barcode must be 13 digit long
ITF barcode must be 14 digit long.
Once filled out, delete the examples and headers and download as a CSV.
Move to your NAV desktop and import to NAV via the "Import Item Cross Reference" button on the item page (see screenshot below).