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

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "attributes": {
      "api_slug": "<string>",
      "display_name": "<string>",
      "description": "<string>",
      "reference_type": "<string>",
      "validation": {},
      "default_format": "<string>",
      "default_value": "<string>"
    }
  },
  "links": {
    "self": "<string>"
  }
}