Custom field on customer card "Ship to Address" to show on SO shipping tab

We have a custom field on the customers address form which houses the GLN for that address. I need to pull the custom field into the SO screen, but this field needs to be dynamic and update according to the shipping address selected on the sales order.

I assume a workflow is required to add this field to the Sales order, the custom field (which is an "other record field") was created as a free-form text type and store value was selected. I updated the Validation & Defaulting default value to "GLN" and have ticked formula.

I've seen the instruction to pull in the state or zip code of the shipping address on the sales order into a custom field (and have been able to do this with a workflow) but as this custom field isn't showing in the "shipping address" field on the SO I can't figure out how to pick it up to pull into my SO custom field.