400/404/422 Errors Trying to Create a Fulfillment

I am trying to fulfill an order via the API with this Javascript code:

I know my token/config and URL are correct as I have successfully integrated 4 other APIs this morning using the same code.

Depending on how I format my object, I’ll receive either a 400, 404, or 422 error back. I’m not sure which one best indicates I have the object/JSON structure correct.

If I understand the Shopify response to some of these tickets, order_id and fulfillment_order_id are different.

I have the ID for both the order and the line_item although I don’t have a fulfillment_order_id because of course I am attempting to create a fulfillment.

That may be the part I’m most confused about. This page in the docs is indicating how to “create” a fulfillment record but there is already somehow an ID in their example?