add custom field to invoice/packing slip woocommerce POS

We need to add in to the packingslip the location number from each product. The location number is a custom field added also by functions.php

I found the below and how to add product attributes for each item and that works but i can not work out how to add a custom field for each product.

https://docs.wpovernight.com/woocommerce-pdf-invoices-packing-slips/displaying-product-attributes/
https://docs.wpovernight.com/topic/woocommerce-pdf-invoices-packing-slips/