@Philip That will depend on whether your users are synced from an AD Connect or Cloud Only (created directly on Azure AD).
If your tenant users are synchronized from on-premises Active Directory, use "Contoso\username".
If your tenant users are created in Azure AD, use "AzureAD\UserUpn"
This document has more information about managing local administrator accounts on your AAD Joined devices: https://learn.microsoft.com/en-us/azure/active-directory/devices/assign-local-admin#manually-elevate-a-user-on-a-device