Hello, thank you for reaching out to Microsoft Q&A.
Do you mean user email notifications or developer API notifications in Outlook Web?
If you would like to turn on email notifications in Outlook Web, (user email notifications),
Paste the link below into your browser’s address bar and press Enter:
https://outlook.live.com/mail/0/options/general/notifications
Or, you can navigate manually:
Go to Settings > General > Notifications in Outlook.com on the web.
From there, you can turn on notifications for incoming Outlook emails, calendar events, and documents.
Let me know if you still need more help.