That’s not really possible. Bubble automatically handles login and sign up with oauth. But my setup for one of my apps is (it runs on Google oauth though – but i imagine its pretty similar) is that the OAuth consent only allows for people within my org. It doesn’t really matter to me if existing people in my org “login with google” and it creates them an account because they cant view anything by default without having perms added.
So you’ll have to still allow them to log in with email + pw, then force them upon next login to sign in with Microsoft so the next time they can just sign in with Microsoft