code: public RtmStartResponse rtmStart(String accessToken) throws IOException {
      HttpUrl url = baseUrl.newBuilder("rtm.start")
          .addQueryParameter("token", accessToken)
          .build();
      Request request = new Request.Builder()
          .url(url)
          .build();
      Call call = httpClient.newCall(request);
      try (Response response = call.execute()) {
        JsonAdapter<RtmStartResponse> jsonAdapter = moshi.adapter(RtmStartResponse.class);
        return jsonAdapter.fromJson(response.body().source());
      }
    }
docs: See https://api.slack.com/methods/rtm.start.