API Fulfillment availability for primary domain

[NicolasIPSO]

Hi, we made API development for a client and we have a problem.  Our client has a primary domain that is the live and there is the test-site domain that redirect to the primary domain. Both domains are for the same shop. 

We are able to get the orders on any of the 2 domains. 

We are getting a "Not found" error on the fulfillment API when using the active domain, but it works fine if we use the test-site url. 

How can we get the APIs used in our application to work on any domains of a single shop (well, preferably on the primary domain)

Here is basically what we get (an empty request was used here for test purpose). 

https://www.mydomain.co/admin/api/2024-01/fulfillments.json    

{
    "errors": "Not Found"
}
https://mydomain-co-test-site.myshopify.com/admin/api/2024-01/fulfillments.json  
{
    "errors": [
        "Fulfillment payloads fulfillment order must be greater than 0"
    ]
}

[YOD_Solutions · Accepted solution]

I don't think you can use a custom domain for the Shopify API url. It should always be ".myshopify.com" domain.

[NicolasIPSO]

That's also what I think I understood from other forum comments. What it looks like is that GET calls are going through fine but that POST request are not going through. Using the .myshopify.com domain absolutely works in every case so that's what we will use. 

Thank you.