If you are looking API for mailbox usage reports. you can use Graph APIs endpoint such as
GET /reports/getMailboxUsageDetail(period='{period_value}')
For more details, please find the documentation Mailbox usage reports , Get details about mailbox usage.
This can be tested via graph explorer .
If you want to register AAD (Microsoft Entra ) app., follow the this document.
afterward, you have to generate token for executing the graph API by consenting to the app.
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