I am trying to hit the (unsupported) https://main.iam.ad.ext.azure.com/api/Directories/Properties endpoint. I would like to authenticate an Application that I have created under App registrations.
My Application has the following API Permissions:

I have created an access token by sending a POST request to https://login.microsoftonline.com/{tenantId}/oauth2/token?api-version=1.0, with the following body:
grant_type=client_credentials
client_secret={clientSecret}
client_id={clientId}
resource=https://main.iam.ad.ext.azure.com/
Using this access token, the https://main.iam.ad.ext.azure.com/api/Directories/Properties endpoint responds with 401 - Unauthorized: Access is denied due to invalid credentials.
What API Permissions & token request do I need to make in order to authorize this service principal to fetch resources from this endpoint?