Computer Configuration > Administrative Templates > Microsoft Edge > Extensions
Both
HKLM\Software\Policies\Microsoft\Edge
This policy lets you specify a list of extension IDs that must be explicitly allowed by the user to run in InPrivate mode in order to enable InPrivate browsing. If users don't allow all listed extensions to run in InPrivate mode, they'll be unable to navigate using InPrivate. If any extension in the list isn't installed, InPrivate navigation is blocked. This policy only applies when InPrivate mode is enabled. If InPrivate mode is disabled using the InPrivateModeAvailability policy, this policy has no effect. Example value: abcdefghijklmnopabcdefghijklmnop