Error "Client Not Trusted; Cause: JSON Server error HTTP 403" is most likely due to the fact that trust relationship between TWX and Windchill servers was not setup correctly. Trust can be established using either user name and password or secure keys.
I am not sure what is your service implementation but take a look at the ContentLoaderFunctions in ThingWorx for examples on how to make a REST call from TWX. Make sure to pass proper authorization parameters to authenticate the request. If you still get an error take a look at Windchill method server and apache access logs for clues why the request failed.