Hello!

It seems you are referring to the JWT token App Secret option. This will only work if you have your own Jitsi Meet server with the JWT tokens feature enabled. If you simply enable it in the Messenger settings, you won't be able to connect to the server, which is exactly what you are experiencing now.

The UNA 333Cc6870758486893 is a unique identifier for the chat that has just started. It is not server-related information. It also allows you to share a link to the current chat for global connection through your Jitsi server, provided the chat is open.