Some data only exists on the Creator side — a tax identifier captured at signup, a project code, an internal reference. This pattern lets Zoho Books push an invoice into Creator by webhook, then writes the missing value straight back onto the Books invoice as a custom field. Two systems, one round trip.

Before you start

• A webhook in Zoho Books (Settings → Automation → Webhooks) pointing at your Creator form’s API endpoint.
• A Zoho OAuth connection named zoho_books_connection with ZohoBooks.invoices.UPDATE.
• An Invoices form with Invoice_ID, DNI_NIE and CIF.
• The custom field already created in Books, and its index.

Deluge script

// ── On Add > On Validate ─────────────────────
// Zoho Books webhook posts the invoice into Creator.

// 1. Reject duplicates from webhook retries
if(Invoices[Invoice_ID == input.Invoice_ID].count() > 0)
{
    cancel submit;
}

// 2. Require at least one tax identifier
taxId = "";
if(input.DNI_NIE != null && input.DNI_NIE != "")
{
    taxId = input.DNI_NIE;
}
else if(input.CIF != null && input.CIF != "")
{
    taxId = input.CIF;
}

if(taxId == "")
{
    cancel submit;
}

// 3. Push the tax id back onto the Books invoice as a custom field
customField = Map();
customField.put("index",2);
customField.put("label","NIF / CIF / NIE");
customField.put("value",taxId);

customFields = List();
customFields.add(customField);

updateMap = Map();
updateMap.put("custom_fields",customFields);

resp = zoho.books.updateRecord("invoices","YOUR_BOOKS_ORG_ID",input.Invoice_ID,updateMap,"zoho_books_connection");

if(resp.get("invoice") == null)
{
    info "Books update failed: " + resp.toString();
}

Notes

• The duplicate check is not optional. Webhooks retry. If the first delivery succeeded but the acknowledgement was lost, Books sends it again — and without the count check you get two records for one invoice. This is the single most common webhook bug.
• On Validate, so you can still refuse. cancel submit only works before the record is written. On Success is too late.
• Custom fields are matched by index. The index is the field’s position in the Books custom-field list, and it is easy to break by reordering fields later. Some endpoints accept customfield_id instead, which is stable — prefer it if your call supports it.
• Module names are lowercase. "invoices", not "Invoices". The error you get for the wrong case is not helpful.