Hello,
The AI answer is showing you how to add Yahoo in the Outlook app.
But looks like you are asking how to add Yahoo address to your Outlook account, as an alias, not in Outlook App, is that right?
If so, Go to your personal account via this link: https://account.live.com
Go to Your Info > Sign in preference, Add an alias.
Add existing: this will allow you to add a 3rd party email address, such as yahoo, gmail, as long as they are not already associated with another Microsoft account.
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However, you do need to make sure this Yahoo address is not already associated with another Microsoft account. If it is, you want to give that Microsoft account a different alias first, then delete the Yahoo address from there, so that you can bring it over to the Microsoft account you want to add to.