You make a dummy booking by doing the search for your dates/ route so that it displays the fares (and flight options) so you can see how much they cost but you don't progress onto the confirmation pages and just end the session or use the 'back' button.
You can make a provisional reservation which they will hold for 24 hours but that has to be done over the phone - if you don't call back in the 24 hr period it lapses. I think they will hold it for a further 24 hours (but that is the limit) but you have to phone within the 1st 24hr period.
It is not a facility available over the internet
BTW ignore the VS bull about APD. The Government won't get rid of it or lower it and it is just a smokescreen that VS use to hide their own exhorbitant fuel surcharge.