Is this for SfB Server or SfB Online? The reason why i ask is SfBO supports dynamic conference ID's by default now. This is a server side driven, tenant wide setting though, which can be turned on/off from powershell. It also only applies to tenants/users enabled for Microsoft PSTN Conferencing too.
When using SfBO there are no client side options to control the use of dynamic or static conference IDs in Outlook (This applies to Windows or Mac) when using Dynamic conference IDs. This is by design if dynamic conference IDs are in use. That said im not sure if the latest Outlook for Mac build is able to control the use of Dynamic/Static Conference IDs (Private or Public meetings) as im running the latest build against SfBO. This may differ if using Outlook for Mac against SfB Server though...
You can change the default meeting configuration to private in SfB Server via the Set-CsMeetingConfiguration -AssignedConferenceTypeByDefault $false but this is a server side global/site/pool wide setting. But this setting will affect all Outlook clients regardless of wether they're a Mac or on Windows. Sorry for the lengthy response but as you can appreciate there are a few nuances/variances between Server, Online and the Outlook client on Mac and Windows.
