curl --request POST \
  --url https://api.ctrl-hub.com/v3/operations/{operation_id}/relationships/assignees \
  --header 'Content-Type: application/vnd.api+json' \
  --header 'X-Session-Token: <api-key>' \
  --data '
{
  "data": [
    {
      "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "type": "<string>"
    }
  ]
}
'

{
  "id": "98ca4a78-b66f-4234-9719-aaf832ee6669",
  "status": "400",
  "title": "A validation error was encountered",
  "source": {
    "parameter": "include"
  },
  "meta": {
    "resource": "wrong_value"
  }
}