Sorry, are you trying to create services via the REST API? That would be complicated. Any custom services can be accessed via REST API as a POST request. You just need to write a custom service for this. There are snippets in the service editor that help you convert from JSON to infotable and vice versa.
Here is some documentation on it: https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS179012
Hope this helps!
All of our services use the POST method in ThingWorx, with very few exceptions. There is an overview on using the ThingWorx REST API here: https://community.thingworx.com/community/developers/blog/2016/06/15/rest-api-overview-and-examples?&art_lang=en&posno=1…
I am not sure why it should matter where this service is located. What is the service doing? Where are you calling it from?