You may need to create a new folder and add it to a structure. 

Folders and generators are items that are managed entirely by Structure add-on, so you'll need to use Structure's services to create the item first, giving you the item identify, and then insert a row into a forest.

Read more about Changing Structure Content for general ideas about updating a structure.

1. Create the Folder entity

long folderId = myStructureComponents.getFolderManager().createFolder(Folder.named("My Stuff").build());

The folder is now stored in the database.

2. Define folder's identity

ItemIdentity itemId = CoreIdentities.folder(folderId);

3. Add folder to structure

forestSource.apply(new ForestAction.Add(itemId, 0, 0, 0));