What you can do is to edit the optional claims in the Enterprise Application (Service Principal) which is connected to the App Registration. In the Enterprise Application you can configure optional claims that should be passed to ID and access tokens.

1. Go to the Enterprise Application you want to edit
2. Select Single sign-on
3. Edit Attributes & Claims

4. Click on Add new claim

5. Add your specific claim and Save

Afterwards you should see the optional claim in the Enterprise Application and should get your user.extensionattribute15 as your configured claim in the ID and access token.