public interface FacebookAppLoginService {
    @POST("app/fbapp_login")
    Observable<User> getUser(@Body FacebookAccessToken facebookAccessToken);
}

Retrofit retrofit = new Retrofit.Builder()
            .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
            .addConverterFactory(GsonConverterFactory.create())
            .baseUrl("http://rocky-garden-56471.herokuapp.com/")
            .build();

FacebookAccessToken facebookAccessToken = new FacebookAccessToken();

Observable<User> newUser = facebookAppLoginService.getUser(facebookAccessToken);

newUser.subscribeOn(Schedulers.newThread())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(userInfo -> {
                        Log.i("Server Call", "Received: " + userInfo.toString());
                });