Hello @Ning Wang ,
At this point this functionality to interact with tenant object at this level is only available to some apps internally . Currently you will need to do this via Azure portal , CSP portal or Office 365 portal which creating a tenant . This functionality is not exposed through Microsoft Graph. A few similar requests have been filed in user voice. Please check the same.
https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/38740858-create-azure-ad-programmatically
https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/38699395-delete-directory-programatically
https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/38743126-programatically-provision-and-manage-aad-b2c-tenan
I would suggest you to upvote and provide your requirements on the related uservoice feature request . The uservoice feedback portal is periodically reviewed by the product group for prioritizing new features based on customer demand, feasibility and other factors. Hope this helps provide the clarification.
Thank you.