Modifying a Custom Field (DataExt)

You have to add the custom field either via the QuickBooks UI, or with a DataExtDefAdd request before you can add or modify the data contained within it.

<?xml version="1.0" encoding="utf-8"?>
<?qbxml version="7.0"?>
<QBXML>
	<QBXMLMsgsRq onError="stopOnError">
		<DataExtModRq>
			<DataExtMod>
				<OwnerID>0</OwnerID>
				<DataExtName>MyCustomerNumber</DataExtName>
				<ListDataExtType>Customer</ListDataExtType>
				<ListObjRef>
					<FullName>Keith Palmer</FullName>
				</ListObjRef>
				<DataExtValue>1234</DataExtValue>
			</DataExtMod>
		</DataExtModRq>
	</QBXMLMsgsRq>
</QBXML>