AKS BYO Private DNS Zone no assign identity parameter?

I can use az aks cli to create a BYO Private DNS zone:

az aks create -n xx -g xx --load-balancer-sku standard --enable-private-cluster --enable-managed-identity --assign-identity ResourceID --private-dns-zone ResourceID

But i dont see the assign identity parameter as part of the terraform provider azurerm_kubernetes_cluster

The example shows to add two parameters only:

  private_cluster_enabled = true
  private_dns_zone_id     = azurerm_private_dns_zone.example.id

and misses the important 3rd parameter required which is the assigned identity to the DNS zone

Have i got this wrong?  Throws me an error when i execute my code regarding DNS

Error: a user assigned identity or a service principal must be used when using a custom private dns zone

Any help appreciated