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));