There's a requirement to apply AD permissions to the PMAdmin / Helpdesk site's.
To prevent information shown between domains such as management policies etc.
Being able to restrict access to the Helpdesk scope's
An enhancement request (TF00680321) has been created detailing the feature above.
The product team will evaluate the request and this feature may become available on a future release of the product.
Please refer to this article for updates or contact support referencing the Enhancement Request ID: TF00680321.