Hi Libin Mathew,
Thanks for reaching out to Microsoft!
To use application permission for this API, tenant administrators must create an application access policy and grant it to a user. Allow applications to access online meetings on behalf of a user which I believe is already set up for your tenant as per the details shared.
Please try to use the API in Use Postman with the Microsoft Graph API as you are using application permissions. Additionally, switch from the beta version to v1.0.
Note: Usually, it is not recommended to use the Beta versions of API in production as they are subject to change according to Microsoft policies and they can be leveraged only when they are available for general use.
Hope this helps.
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