code: public FolderJob createFolder(String folderName, Boolean crumbFlag) throws IOException {
          // https://gist.github.com/stuart-warren/7786892 was slightly helpful
          // here
          //TODO: JDK9+: Map.of(...)
          Map<String, String> params = new HashMap<>();
          params.put("mode", "com.cloudbees.hudson.plugins.folder.Folder");
          params.put("name", folderName);
          params.put("from", "");
          params.put("Submit", "OK");
          client.post_form(this.getUrl() + "/createItem?", params, crumbFlag);
          return this;
      }
docs: Create a folder on the server (as a subfolder of this folder)
  
  @param folderName name of the folder to be created.
  @param crumbFlag true/false.
  @return
  @throws IOException in case of an error.