Are you sure you need the RestServices module? That's that "old way" of consuming REST services in Mendix. Instead, you might try the native REST functionality in Mendix:
Good luck and let us know if you have any questions!
In addition to Eric's post: I'd only use the module to set up a REST endpoint.
For testing you can use Postman (https://www.getpostman.com/), very user-friendly and flexible!