@Martijn van Os | KidsKonnect ,Thanks for posting in our Q&A. From your description, I know we have connected Microsoft account with Apple Business Manager. But when user logs in their MacBook, the authentication entered without logging. If there's any misunderstanding, please let me know.
Before going on, I would like to confirm if connecting Microsoft account to ABM means we use federated authentication to link Apple Business Manager to Microsoft Azure AD. Here is a link for the reference:
https://support.apple.com/en-sg/guide/apple-business-manager-m/apdb19317543/web
Or if we means we are enrolling macOS devices with ABM. Here is a link for the reference:
https://learn.microsoft.com/en-us/mem/intune/enrollment/device-enrollment-program-enroll-macos
Please help confirm the above information and thanks for your time.
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