For the owner (3CX user/extension) of the meeting, this is not possible in any of the two "modes" as they require a unique URL to authenticate and become by default moderator of their room.
If you require a static room for a group of people, another option is to create a scheduled meeting and place it in the far future. The meeting room is already created now and is accessible for each user by their invitation URL or for ad-hoc user by using the "join URL" from within the meeting.