That functionality doesn’t exist yet but is being worked on. Currently in ISA/Invest, if you try to place an order it will just be rejected. Need to wait until trading begins. Or at least until a price is applied to the instrument.
@Rygel, these aren’t IPOs, they are Direct Listings, so I don’t think we’ll have to wait for anything once they are live. If anything this might be a bit like dealing in an AIM stock where you need to have enough sellers in the market to cater for all the buyers.