Australia: drive-away prices already carry the GST
GST at 10% is already inside the advertised drive-away price, so there is no tax to add and no field to fill in. The distinction matters when you are comparing quotes: a drive-away figure is what you finance, while a price quoted before on-road costs is not, and the difference is not a tax the calculator can work out for you. Enter the drive-away number.
The one charge that can sit outside this model is Luxury Car Tax, which applies above a threshold on the value of the car and is not modelled here. If the vehicle you are pricing is over the threshold and the quote you have does not already include LCT, add the amount to the fees field so it is financed with everything else rather than quietly missing from the payment.
A trade-in in Australia reduces what you borrow by its equity, but it cannot reduce a GST that was never itemised on the invoice as a separate line you are being charged.