Basil: How do I share just a specific subfolder in the External folder?

This article is intended users under the Basil Advanced plan. Basil Simple users can refer to this article.
You can only grant shared access at the Workspace level of a Client rather than at a subfolder or file level.
You can instead create a separate Workspace for the Client, move any files there, and invite a user to that Workspace as Client User (not Primary User).