curl --request GET \
  --url https://api.terminal49.com/v2/tracking_requests/{tracking_request_id}/custom_fields \
  --header 'Authorization: <api-key>'

{
  "data": [
    {
      "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "type": "custom_field",
      "attributes": {
        "api_slug": "<string>",
        "value": "<string>",
        "display_value": "<string>"
      },
      "relationships": {
        "entity": {
          "data": {
            "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
          }
        },
        "definition": {
          "data": {
            "type": "custom_field_definition",
            "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
          }
        }
      }
    }
  ],
  "links": {
    "last": "<string>",
    "next": "<string>",
    "prev": "<string>",
    "first": "<string>",
    "self": "<string>"
  },
  "meta": {
    "size": 123,
    "total": 123
  }
}