Custom Fields - Credit Note

Specific to CreditNotes, the name the be use should contain "CreditNote" and not "Invoice". Below an example with a custom field on the second detail line:

{
 "OrderType": "CreditNote",
 "OrderDirection": "Income",
 "OrderNumber": "QS-9424",
 "OrderDate": "2025-06-15",
 "ExpiryDate": "2025-08-30",
  "AboutInvoiceNumber": "QS-9116",
 "CustomFields"  : {
    "CreditNote.InvoicePeriod.StartDate": "2025-05-01", 
    "CreditNote.InvoicePeriod.EndDate": "2025-05-30", 
    "CreditNote.Note": "header note 1234 ",
    },
  "Customer": {
   "Name": "Billit",
   "VATNumber": "BE0563846944",
   "PartyType": "Customer",
   "Addresses": [
        {
        "AddressType": "InvoiceAddress",
        "Name": "Billit",
        "Street": "Oktrooiplein",
        "StreetNumber": "1",
        "City": "Ghent",
        "CountryCode": "BE"
        }  
   ]
   },
 "OrderLines": [
    {
   "Quantity": 2,
   "UnitPriceExcl": 3.75,
   "Description": "Sticks",
      "VATPercentage": 21,   
   "CustomFields": {
   "CreditNoteLine.Note": "Your ref: TW-169944  Our ref: 25001377"
   }
   }
 ]
}