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