Hi there.
Yes I did add http://take.ms/Aibv5
Here is the plugin: https://docs.woocommerce.com/document/woocommerce-admin-custom-order-fields/
Another issue I noticed:
We have have 3 shops, and in the US one the “po” field appears in the drop list, but in the Mexico shop it does not.
US: http://take.ms/8uvHv
Mexico: http://take.ms/rwfZF
Thank you for your support!