As with most people that have their email for years or decades, my email address have been leaked too many times and probably traded for less than a tenth of a cent in dark web. Email account now is more important than ever due to online digitalization of our lives. Losing access or some bad actors gaining access to your email account can even causes all your money to be stolen.
Outlook can show you the login activity of your account and honestly it is depressing to see so many login attempts by some bots as the picture below.

I got more than 10 login attempts everyday! btw Google is a pecker for not showing login attempts logs.
As you can see the last unsuccessful attempt is on the 30th Dec and there's no more unsuccessful attempt after that. That is when I created the alias. So here I make a guide on how to do it.

Login into outlook.com. Once you in the email, as image above shown, first go to top corner and click "My Microsoft account"

Then click on "Your info" on the left side and then click "Edit account info"

After the page load it will show your current primary email, click on "Add email"

Put some creative name for your alias if this is the email you'll use for login and not for corresponding with others, then hit the "Add alias" button. Creative name will help to avoid being guessed by bots.

Once created, it will now show 2 email addresses. Click on "Make primary" to make it as your main account. Your account's primary alias is displayed on your Microsoft devices (such as a Windows PC, Xbox, or Windows Phone)
Right now both email can be use as ID to login into the account. Need to follow below steps to disable login with old email.
And don't be a smartass and choose the "Remove" on old email address, doing so will delete all the email you receive from old email address.

Still at the same page, click on "Change sign-in preferences"

Untick on the old email address and click "Save"
And that's it! Your old email cannot be used to login to the account, it still can receive and send email as before but you need to use the new email as ID to login.
By doing these, hacker or bot can't even try to hack your account as they doesn't know your login email. You can also create additional aliases for sort of burner account, useful when some service need an email to signup or register but you don't want it to start spamming you with useless email or worse sell your email address info to telemarketers, scammers or dark web.