When you have written the function in the backend, you can test your function call and view the result that comes back from the server using the Trigger Backend functions feature, a powerful tool hidden behind a small green arrow. If you’re familiar with Postman, the API testing tool, this is a utility to test your backend functions while still in the editor.
For your function, you would write the wix-fetch code that speaks to the 3rd party server within your exported function, and test it from there. You can use this tool to debug the responses you get back.