Siri on iOS can only create calendar events in accounts that are connected to the iOS system Calendar. It cannot create events directly in the standalone Outlook app without that account being added to iOS.
To have Siri-created items go into a Microsoft service (Outlook/Microsoft 365/Exchange), the Microsoft account must be added to iOS and used as the default for that type of item. This is the same model used for Microsoft To Do and Reminders integration, where the Microsoft account is connected through iOS settings rather than directly to the app.
Because of this design, creating Outlook calendar events via Siri without connecting the Microsoft account to the iPhone’s Calendar app is not supported.
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