Adding a subtab below QC tab in Purchase order

To add a ‘QC Requirements’ subtab below QC tab in purchase order.

                var sublist = form.addSublist({
                    id: 'custpage_sublist',
                    type: serverWidget.SublistType.LIST,
                    label: 'QC Requirements',
                    tab: 'custom1660'
                });
                sublist.addField({
                    id: 'customfield1',
                    type: serverWidget.FieldType.TEXT,
                    label: 'Line #'
                });
                sublist.addField({
                    id: 'customfield2',
                    type: serverWidget.FieldType.TEXT,
                    label: 'Item'
                });
                sublist.addField({
                    id: 'customfield3',
                    type: serverWidget.FieldType.TEXT,
                    label: 'QC Questions'
                });
                sublist.addField({
                    id: 'customfield4',
                    type: serverWidget.FieldType.TEXT,
                    label: 'CRFQ'
                });

add