RetroClientAuth.java

    OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();

    //Create a new Interceptor.
    Interceptor headerAuthorizationInterceptor = new Interceptor() {
      @Override
      public okhttp3.Response intercept(Chain chain) throws IOException {
        okhttp3.Request request = chain.request();
        Headers headers = request.headers().newBuilder().add("Authorization", authToken).build();
        request = request.newBuilder().headers(headers).build();
        return chain.proceed(request);
      }
    };
    //Add the interceptor to the client builder.
    clientBuilder.addInterceptor(headerAuthorizationInterceptor);

    return new Retrofit.Builder().baseUrl(ROOT_URL)
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        .addConverterFactory(GsonConverterFactory.create())
        .build();