Creating a metadata field in orders

Site URL: https://www.maisonlouno.com

Hi! I'm starting to work with a shipping center for my business. The shipping center has managed to connect their API on my Squarespace site, but miss one last step which the Squarespace support team can't help me with: creating a new metadata field on orders. The aim of the field is to give status whether or not an order has been transferred to the shipping center API. This way we can check in case an order isn't transferred. The field must be changeable by API.

Please can any of you help with this? Where to change in the code? What to write?

Many thanks!! Louise