The Space ID is used to access your organization's Backlog Space. It can be found in the subdomain part of the Space URL.
It is also included in the email sent to the Space owner when the Backlog Space is registered.
If you have been invited to a Backlog Space, you can simply follow the Space URL in the invitation email.
If you have forgotten your Space ID, please contact the Space Owner, Administrator or Project Administrator who invited you.