curl --request POST \
  --url https://restapi.fax.plus/v3/accounts/self/ai/custom-fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "name": "Invoice Number",
  "instructions": "Extract the invoice or reference number from the document",
  "enabled": true
}
'

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