code: protected int addPayload(RestRequestBuilder builder, String payload) {
      if (payload == null || payload.length() == 0) {
        return 0;
      }
  
      builder.setHeader(RestConstants.HEADER_CONTENT_TYPE, RestConstants.HEADER_VALUE_APPLICATION_JSON);
      try {
        DataMap data = JACKSON_DATA_CODEC.stringToMap(payload);
        byte[] bytes = JACKSON_DATA_CODEC.mapToBytes(data);
        builder.setEntity(bytes);
        return bytes.length;
      } catch (IOException e) {
        throw new RuntimeException("Fail to convert payload: " + payload, e);
      }
    }
docs: Add payload to request. By default, payload is sent as application/json