code: private String passthroughRequest(HttpMethod method, String endpoint, String payload,
                                        HashMap<String, Object> parameters) throws SmartsheetException {
          Util.throwIfNull(endpoint);
          Util.throwIfEmpty(endpoint);
  
          if(parameters != null)
              endpoint += QueryUtil.generateUrl(null, parameters);
  
          HttpRequest request = createHttpRequest(smartsheet.getBaseURI().resolve(endpoint), method);
  
          if(payload != null) {
              HttpEntity entity = new HttpEntity();
              entity.setContentType("application/json");
              entity.setContent(new ByteArrayInputStream(payload.getBytes()));
              entity.setContentLength(payload.getBytes().length);
              request.setEntity(entity);
          }
  
          String res = null;
          try {
              HttpResponse response = this.smartsheet.getHttpClient().request(request);
              switch (response.getStatusCode()) {
                  case 200:
                      String readLine;
                      try {
                          BufferedReader br;
                          StringBuilder sb = new StringBuilder();
                          br = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
                          while ((readLine = br.readLine()) != null) {
                              sb.append(readLine);
                          }
                          br.close();
                          res = sb.toString();
                      }
                      catch (IOException e) {
                          res = null;
                      }
                      break;
                  default:
                      handleError(response);
              }
          } finally {
              smartsheet.getHttpClient().releaseConnection();
          }
          return res;
      }
docs: Passthrough request
  
  @param method HTTP method
  @param endpoint the API endpoint (required)
  @param payload optional JSON payload
  @param parameters optional list of resource parameters
  @return the result string
  @throws SmartsheetException