[DEPRECATED] Creates a Custom Field

curl --request POST \
  --url https://api.rootly.com/v1/custom_fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "custom_fields",
    "attributes": {
      "label": "<string>",
      "description": "<string>",
      "shown": [
        "incident_form"
      ],
      "required": [
        "incident_form"
      ],
      "default": "<string>",
      "position": 123
    }
  }
}'

{
  "data": {
    "id": "138",
    "type": "custom_fields",
    "attributes": {
      "slug": "test-custom-field",
      "description": null,
      "enabled": true,
      "position": 11,
      "updated_at": "2025-07-30T02:08:53.440-07:00",
      "created_at": "2025-07-30T02:08:53.440-07:00",
      "kind": "text",
      "label": "Test custom field",
      "shown": [
        "incident_form",
        "incident_slack_form"
      ],
      "required": []
    },
    "relationships": {
      "options": {
        "data": []
      }
    }
  }
}