Walkthrough 3-5: Proxied requests cannot be made to localhost for POST

Hello,

I am using the PDF for the December and later walkthroughs, and for walkthrough 3-5, when i attempt to call the POST for the American Flights API, the response I get is a 400 with the message "Proxied requests cannot be made to localhost." This works fine through the API Designer, as it looks like the request is not going through a proxy there. Is there anything i need to do for this?

Thanks,