The MainActivity demonstrates how to POST a json body to a REST API endpoint and retrieve json results.
The initial project accesses Relative Locationing with pre-build JSON body. This body is normally generated from scanning BSSIDs/WiFi. But that code is not included, and will need to be developed.
The button.setOnClickListener code can be copied, added then modified for many endpoints.
It uses okhttp-3.11.0.jar found here: http://square.github.io/okhttp/