CORS issue.

When I use any "core" API I am successful (example): https://public.billsby.com/api/v1/rest/core/...

When I try to use the api from "billing" (as shown here) https://public.billsby.com/api/v1/rest/billing/...
I get the CORS error (shown below) :
Access to fetch at 'https://public.billsby.com/api/v1/rest/billing/[companyName]/subscriptions/[subscriptionId]/addons' from origin 'http://localhost:65508' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

I used the APIs "Try It" function, which works on the billsby site. However, copying it directly into my scripting area, still produces the same error shown above. Please advise.