curl --request POST \
  --url https://api.{tenant}.getomneo.com/api/v3/transactions/{transaction}/custom-fields

{
  "data": {
    "name": "<string>",
    "handle": "<string>",
    "namespace": "<string>",
    "value": "<string>",
    "type": "<string>",
    "custom_fieldable_type": "<string>",
    "custom_fieldable": {
      "id": "<string>",
      "external_id": "<string>",
      "profile_id": "<string>",
      "meta": "<string>",
      "total": "<string>",
      "total_original": "<string>",
      "rounding": "<string>",
      "margin": "<string>",
      "is_void": "<string>",
      "transacted_at": "<string>",
      "timezone": "<string>",
      "payments": "<string>",
      "receipt_is_email": "<string>",
      "receipt_ref": "<string>",
      "linked_receipt_ref": "<string>",
      "claimed_at": "<string>",
      "receipt_email": "<string>",
      "currency_id": "<string>",
      "currency_rate": "<string>",
      "currency": "<string>",
      "type": "<string>",
      "status": "<string>",
      "order_number": "<string>",
      "order_id": "<string>",
      "external_order_id": "<string>",
      "created_at": "<string>",
      "updated_at": "<string>"
    },
    "created_at": "<string>",
    "updated_at": "<string>",
    "version": 123,
    "is_index": 123
  }
}