According to the Autodesk Forge references, a folder is "a logical organization of items within a project". So that means that a folder contains several items.
But the webservice to create an item looks like that:
POST /data/v1/projects/{project_id}/items
This web-request only takes the project-id into account. But there seems to be no way, to provide the folder-id. So how can I create an item in a specific folder?