The uuid4 can be received by making a GET against the connector /v1/connectors?filter=name,like,%22%25ft%25%22 where `ftp` is the connector name.
To use this authentication in a white label flow, pass it's uuid4 you receive in the response as part of the query string as per the example here. So ideally, after creating, you store it on your customer's account or company object in order to use it for all automation flows.
To update, PUT /v1/connector-auths/**uuid4**
"connector_id":"some uuid4 of the corresponding connector",