To enable 2-step authentication/verification:
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Go to the Outlook.com website and log in.
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Select the gear icon.
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Go to Options > Account details (top of the list). You may be asked to sign in.
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After the account.live.com page has opened, select Security & Privacy, go to More Security Settings, scroll down and select Set up two-step verification.
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You'll have the option to confirm your identity via email, phone or the Microsoft account App. After you've completed this step, you'll need to use an app-specific password to log in via the Outlook App.
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Under Security & Privacy > More Security Settings, scroll down and select Create a new app password. Use the newly generated password to log in.
See the image below showing where to go in Security & Privacy.
Find more information on 2-step authentication with other accounts: