For now, preferred Webex site can only be set in the new Webex app if the option for PMR is enabled in CH. Preferred site is needed in order for the users to be able to start/join meetings directly from the app. However, without that, they are missing the start/join buttons. In the old Meetings app, preferred Webex site is not needed to start/join meetings.
Ideally, we are looking for a way to have start/join options in the new Webex app, even if the customer has PMR disabled, which they do due to security measures, and will not turn it on.