code: public int createIncompleteResponse(int surveyId, String token) throws LimesurveyRCException {
          LsApiBody.LsApiParams params = getParamsWithKey(surveyId);
          HashMap<String, String> responseData = new HashMap<>();
          responseData.put("submitdate", "");
          String date = ZonedDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE) + " " + ZonedDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
          responseData.put("startdate", date);
          responseData.put("datestamp", date);
          if (StringUtils.isNotEmpty(token)) {
              responseData.put("token", token);
          }
          params.setResponseData(responseData);
          return callRC(new LsApiBody("add_response", params)).getAsInt();
      }
docs: Create an incomplete response, its field "completed" is set to "N" and the response doesn't have a submitdate.
  
  @param surveyId the survey id of the survey you want to create the response
  @param token    the token used for the response, no token will be set if the value is empty
  @return the id of the response
  @throws LimesurveyRCException the limesurvey rc exception