To configure automatic enrollment of your AVD Windows 10 Multi-Session hosts you will need to enable the “Enable automatic MDM enrollment using default Azure AD credentials” policy setting in either Group policy or local policy.
Set this to Enabled and Select Credential Type to Use – Device Credential.
This policy setting specifies whether to automatically enroll the device to the Mobile Device Management (MDM) service configured in Azure Active Directory (Azure AD). If the enrollment is successful, the device will remotely managed by the MDM service.