code: public String postRequest(String endpoint, String payload, HashMap<String, Object> parameters) throws SmartsheetException {
          Util.throwIfNull(payload);
          return passthroughRequest(HttpMethod.POST, endpoint, payload, parameters);
      }
docs: Issue an HTTP POST request.
  
  @param endpoint the API endpoint
  @param payload a JSON payload string
  @param parameters optional list of resource parameters
  @return a JSON response string
  @throws IllegalArgumentException if any argument is null or empty string
  @throws InvalidRequestException if there is any problem with the REST API request
  @throws AuthorizationException if there is any problem with  the REST API authorization (access token)
  @throws ResourceNotFoundException if the resource cannot be found
  @throws ServiceUnavailableException if the REST API service is not available (possibly due to rate limiting)
  @throws SmartsheetException if there is any other error during the operation