curl --request GET \
  --url https://api.kit.com/v4/custom_fields \
  --header 'X-Kit-Api-Key: <api-key>'

{
  "custom_fields": [
    {
      "id": 1401,
      "name": "ck_field_1401_last_name",
      "key": "last_name",
      "label": "Last name"
    }
  ],
  "pagination": {
    "has_previous_page": false,
    "has_next_page": false,
    "start_cursor": "WzE0MDFd",
    "end_cursor": "WzE0MDFd",
    "per_page": 500
  }
}