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From the Home screen go to Azure Active Directory > Users. You can see a list of all the valid users, it should be filled with around a dozen dummy accounts and your account. Copy paste your or any of the "User Principal Name" in that list to your request link and this time the request will be valid. From the same menu, you can get userPrincipalName and ID via GET Request (Optional).
Getting userPrincipalName and ID via GET Request (optional)
Optionally, you can extract both the "User Principal Name" and their ID, by doing a GET request to the link,
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