You want to generate a new meeting ID as the default (authenticated meetings) and you run the LYNC Server Management cmdlet Set-CsMeetingConfiguration –DesignateAsPresenter None.  However, it appears you still have to choose the “customize” checkbox before it will default to a private meeting.  Without choosing customize it will still create a public meeting (using the static meet ID).  If you click the customize box in the outlook add-in and click remember settings, add-in will create private meeting every time you schedule a meeting.  If you don’t customize the options in the add-in or remove the checkbox it will create a public meeting with static meeting ID.

Using the LYNC Server Management Shell there are two switches for the cmdlet Set-CsMeetingConfiguration that can affect this.

• EnableAssignedConferenceType – Indicates whether users are allowed to schedule public meetings. With a public meeting, the conference ID and the meeting link remain consistent each time the meeting is held. With a private meeting, the conference ID and meeting link change from meeting to meeting. The default value is True.
• AssignedConferenceTypeByDefault – Determines whether new meetings will be configured, by default, as public meetings. Set this value to True to use public meetings by default; set this value to False to use private meetings by default. The default value is True.

If you set EnableAssignedConferenceType to false, there is now, no checkbox to customize settings

When EnableAssignedConferenceType is set to false, outlook will always create a new private meeting with different meeting id.

http://technet.microsoft.com/en-us/library/gg398648.aspx