curl --request GET \
  --url https://restapi.fax.plus/v3/accounts/self/ai/custom-fields \
  --header 'Authorization: Bearer <token>'

{
  "records": [
    {
      "id": "507f1f77bcf86cd799439011",
      "name": "Invoice Number",
      "instructions": "Extract the invoice or reference number from the document",
      "enabled": true
    }
  ]
}