How to add a snapshot schedule to GCP boot_disk via terraform

I'm trying to add snapshot schedule to the boot disk of the vm_instance.

provider "google" {
  project = "xxxxxx"
}
resource "google_compute_instance" "xxxxxx" {
  name         = "xxxxxx"
  machine_type = "xxxxxx"
  zone         = "xxxxxx"

  boot_disk {
    initialize_params {
      image = "???"
    }
  }
  metadata_startup_script = ";;"
  network_interface {
    network = "default"
   }
}      

I know how to add a schedule to an external disk: https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_disk_resource_policy_attachment

But how do you do this with the vm_instance disk ?

Thanks