...
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.
Figure 12: MSFT Overview
Figure 13: MSFT Users
Getting userPrincipalName and ID via GET Request (optional)
...
Feed this GET request with a non-expired access token and if the request is successful, you will be presented with an array of JSON objects that represents all of the user's information, including their "User Principal Name" and ID. It is also important to note, this is where that optional API permission "User.Read.All" is used. For this guide, this method will not be used.
Figure 13: MSFT Users
The JSON Payload
After pasting the appropriate request link, time to feed it the JSON Payload
...