The information in that post is wrong.
If you have your own domain and an Microsoft 365 personal or family subscription, you can add the domain in Outlook.com's Settings (gear icon) > View all Outlook settings > Premium
https://outlook.live.com/mail/0/options/premium/personalemail
But you need to do it ASAP - before the end of Nov.
My instructions above are correct for family and personal accounts.
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Also see this artilce for some useful tips, including how to keep your registrar.
https://www.slipstick.com/outlookcom/outlook-com-custom-domains/
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