QuickApiCall.java

  public void onCreate() {
    // Http client
    OkHttpClient client = new OkHttpClient();

    // Content type
    MediaType JSON = MediaType.parse("application/json; charset=utf-8");

    // Whatever you want to send, here it's json
    RequestBody body = RequestBody.create(JSON, yourJsonString);

    // The request, you can add headers here too
    Request request = new Request.Builder()
        .url(yourUrl)
        .header(name, value)
        .header(name2, value2) // Can remove these headers if not needed
        .post(body)
        .build();

    new Thread(new Runnable() {
            @Override
            public void run() {
                Response response = client.newCall(request).execute();

                // Switch to the UI thread. Not needed for Log.d, but if you want 
                // to print the response on the UI you'll need to be on the main thread
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        // Print the response (or change the UI)
                        Log.d(response.body().string());
                    }
                });
            }
    }
}

Don't forget to add compile 'com.squareup.okhttp3:okhttp:3.6.0' to your app/build.gradle file.

I'll be trying this soon, thanks a lot!