You may need to be more clear.
Does the URL you provided above directly call the backend service, or does it call a CA API Gateway first?
If the former, then you need to understand the interface of the backend service? How does it expect to receive requests? Do you have documentation for it? For starters, most APIs that expect content to be sent via a HTTP POST expect that content to be sent as the body of the message, not as a query string parameter like your example above.
If the latter, you still need to understand the former, but you also need to understand the interface of the gateway service and the gateway service's policy. Can you successfully call the backend service directly? Can you share your gateway service policy? In any case, if you are hitting the gateway first, it does look like the request is being forwarded to the backend services, because the response has a HTTP server header value of "mashery proxy". That's not something that the CA API Gateway would add on it's own.