do you have the Integromat chrome extension by any chance? You can use it to see exactly what you are sending and what you are getting as a response. Can you also check the headers in the response? Maybe there is some additional info there.
From your screenshot it looks like you are getting an empty array as a response, so best guess - the request parameters are not being sent in the expected format, so the query is not returning anything.
Please use POST method as it explained in API documentation. So it is the correct method.
However, 400 request occurs when something wrong happens with request.
So first identify what is wrong with the request. To do so, please follow the step:
Run the module with POST method and check the request body in Devtool (Integromat)
Duplicate the tab and use module “list orders” from shipday to check the request body in Devtool
Compare both the requests and you will get to know what’s wrong with your request.
You can also proceed with “List Orders” module from shipday instead of API call.
I can replicate how the Shipday module works in HTTP but the issue is that it can only find “current” orders. Any orders I schedule in the future doesnt count as an “order” and it cannot be detected. I have booked a consultation with your company because my brain has melted