I was unable to create neither an open extension nor schema extension through another user in Graph API.
(Same response whether I try to use Graph explorer or Postman.)
Not sure if it is possible to create a schema extension using application permission from another user.
Please refer to this.
I succeeded to create an event from another user using my delegated token, but failed to add a schema extension to the created event.
I guess all delegated and application permissions are allowed.
And regarding this document, what is the difference between get an access token and get an access token after admin consent.
I am currently trying to get application token after admin consent, but not sure if it would work.