Yes. Either actor can send a (text-only) chat message at any time.
Note that the technician sees two chat interfaces; their own, and the end user’s on the remote desktop, so they need to type their message in the right box!
The end user’s chat dialog stays visible after the session ends, so the technician can leave a message if the end user is away from their computer at that time. The end user can also review the activity log post-session.