Login your account via Outlook Web, click on your name in the upper right, then View Account then SECURITY INFO on the left sidebar
1) Click on + ADD SIGN-IN METHOD:
2) Choose authenticator app then ADD:
3) Click on I WANT TO USE A DIFFERENT AUTHENTICATOR APP near the bottom:
Then click next.
A QR code will display, open Google Authenticator on your phone, click on + to add an account.
Scan the QR code with Google Authenticator, the account is now added to your Google Authenticator