Please find below the sample script:

info cfso; is the custom field, but return null

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poID = purchaseorder.get("purchaseorder_id");

organizationID = organization.get("organization_id");

poinfo = zoho.books.getRecordsByID("purchaseorders",organizationID,poID,"zbooks").get("purchaseorder");

delivery_date = poinfo.get("delivery_date");

info delivery_date;

cfso = poinfo.get("cf_sales_order");

info cfso;

mplist = List();

mp = Map();

mp.put("api_name","cf_expected_delivery_date");

mp.put("value",delivery_date);

mplist.add(mp);

info mp;

json = Map();

json.put("custom_fields",mplist);

update = zoho.books.updateRecord("salesorders",organizationID,cfso,json,"zbooks");
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Thanks