curl --request GET \
  --url https://api.firma.dev/functions/v1/signing-request-api/signing-requests/{request_id}/custom-fields \
  --header 'Authorization: <api-key>'

{
  "custom_fields": [
    {
      "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "field_name": "<string>",
      "field_label": "<string>",
      "copied_from_template": true,
      "created_at": "2023-11-07T05:31:56Z",
      "updated_at": "2023-11-07T05:31:56Z"
    }
  ]
}