code: public OAuthSession exchangeCode(String code, HttpUrl redirectUrl) throws IOException {
      HttpUrl url = baseUrl.newBuilder("oauth.access")
          .addQueryParameter("client_id", clientId)
          .addQueryParameter("client_secret", clientSecret)
          .addQueryParameter("code", code)
          .addQueryParameter("redirect_uri", redirectUrl.toString())
          .build();
      Request request = new Request.Builder()
          .url(url)
          .build();
      Call call = httpClient.newCall(request);
      try (Response response = call.execute()) {
        JsonAdapter<OAuthSession> jsonAdapter = moshi.adapter(OAuthSession.class);
        return jsonAdapter.fromJson(response.body().source());
      }
    }
docs: See https://api.slack.com/methods/oauth.access.