Terraform: How to create named_port after google_container_cluster resource?

I created a google_container_cluster ...

resource "google_container_cluster" "primary" {
  name = "primary"
  # rest of the config ...
}

... and now want to add a named_port to the managed instance group that is created on the fly by the google cloud platform.

The google_container_cluster resource returns instance_group_urls. But I don't know how to use this output in order to create a named port.