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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.
Figure 12: MSFT Overview
Getting userPrincipalName and ID via GET Request (optional)
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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
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"Content-type" is used so the request knows what type of payload is it using, which for our case is a JSON object, so enter the value "application/json".
Figure 1214: Request Header I
"Authorization" as the name suggests is used to authorize the request, this is where you will use your access token, the value for this request header will be a string in the format "[TOKEN_TYPE] [TOKEN]", the type of token we are using will be a "Bearer" token and you can simply copy paste the token or use a hash variable to extract the token that was stored in the workflow variable.
Figure 1315: Request Header II
Conclusion
And that's it! With a minimum of 2 JSON Tools, you will be able to send an email using the MS Graph API. By the end, the process of your application should look something like the picture below, 2 tools for requesting an access token and sending the email and any additional activity for automating some of the process, like for the demo app, it is for automating creating the message.
Figure 1416: Process Builder of Send Email using MS Graph Demo App
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