Hello @Kießig, Stefan ,
Thanks for reaching out.
This is not supported scenario, because one of the prerequisite for Hybrid Azure AD join is to configure Azure AD Connect", but It's not supported to install Azure AD Connect in a Azure AD Domain Services to synchronize objects back to Azure AD.
Azure AD Connect should only be installed and configured for synchronization with on-premises AD DS environments.
You could use Azure Active Directory (Azure AD) Join, which joins the device with Azure Active Directory and enables users to sign in to Windows with their Azure AD credentials. If Auto Enrollment is enabled, the device is automatically enrolled in Intune and you can manage them in Intune.
To learn more:
Intune enrollment methods for Windows devices: https://learn.microsoft.com/en-us/mem/intune/enrollment/windows-enrollment-methods
Azure AD joined devices: https://learn.microsoft.com/en-us/azure/active-directory/devices/concept-azure-ad-join
Hope this helps
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