We have implemented Microsoft Information Protection (MIP) for email and document classification with the sensitivity labels Internal, Restricted, Public, and Confidential. As part of our information...
Microsoft does not currently support assigning or overriding Purview sensitivity labels programmatically via Outlook VBA or the COM Object Model. You may use the Microsoft Information Protection (MIP) SDK or Graph APIs for programmatic label application.
https://learn.microsoft.com/en-us/purview/sensitivity-labels
https://learn.microsoft.com/en-us/information-protection/develop/overview