So, let me start with the bad news:
O365 / Microsoft deprecated username / password (badly) this year. This left a lot of people with no email access as their accounts / devices were still set to username / password but their Exchange servers / O365 was set to ADAL / Modern Authentication. And here lies the bad news:With Modern Auth / OAUTH you can't just set the username and hope that the user types in their password.
The authentication flow is completely different and, at best, setting the username may remove just one of the two places that a user will be requested for their email address
Now, the good news: Outlook now (if didn't used to be) has a great set of managed app capability: This allows you to configure a whole host of things.
Assuming that you've got the Outlook app added Under Systems Manager > Apps :
I hope that helps. Let us know how you get on.