Thanks a lot! It works like a charm.
For anybody who might be interested in doing it with Terraform, you must construct the URN of the cluster manually because it does not seem to exist:

resource "digitalocean_project" "k8s" {
  name        = "k8s"
  description = "An auto-scalable kubernetes cluster"
  purpose     = "Web Application"
  environment = "Development"
}


resource "digitalocean_project_resources" "k8s-resources" {
  project = digitalocean_project.k8s.id
  resources = ["do:kubernetes:${digitalocean_kubernetes_cluster.k8s-cluster.id}"]
}