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Regarding access token generation using REST API; the Generate cost detail report API uses AD auth token from the audience https://management.azure.comthis is same for calling any ARM APIs. It is not specific to Cost Management APIs.
This doc https://learn.microsoft.com/en-us/rest/api/servicebus/get-azure-active-directory-token#use-postman-to-get-the-azure-ad-token has example of how to generate auth token for Azure AD.
Hope this helps. Feel free to reply if you have any questions!
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