Create the workspace's Sandbox Bank Connection for development and testing.

Creates a sandbox bank carrying example data — accounts, transactions and investment holdings — so you can build against the API without linking a real bank. Unlike the other link endpoints this returns the connection itself: there is no bank login to complete. Sandbox Bank Connections need no active plan and do not count toward the connection limit, the monthly connection-attempt limit or the annual transaction volume; they are capped at one per Workspace. Deleting one also removes its rows from any connected Airtable base or Google Sheet. Returns 201 when created and 200 when the Workspace already had one.