Before you commence, check with the Australian Taxation Office the current withhold tax rate.
On the ribbon go to Tools > Setups > Accounting > Tax Codes, and edit the tax rate as per advised, against the 'W' withholding tax code.
Enter your PO or expense, and use the W tax code as required. Jim2 will calculate the Pcnt % tax as per added against your tax code in setups.
The transaction will report in the GST Session under the Purchases tab. The transaction will appear in the By Tax tab indicating a W tax code.
However, these values do not flow through to the worksheet. Please adjust manually on your worksheet as required before reporting to the ATO.
They are not reported in W4, or any other field on the GST Calculation Worksheet for BAS.