Manage the Local Administrators Group on Azure AD Joined Devices

• If your tenant users are synchronized from on-premises Active Directory, use net localgroup administrators /add "Contoso\username".
• If your tenant users are created in Azure AD, use net localgroup administrators /add "AzureAD\UserUpn"

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/devices/assign-local-admin