You need to tick the “reverse charge on services” box under the additional VAT options at the bottom of the purchase, as shown by Steve - there’s no distinction between services purchased from the EU vs the US or anywhere else outside the UK, if the “place of supply” is the UK (which it most likely will be in this case, though it gets muddy if your business also operates overseas) then it is subject to UK VAT and needs to be reverse charged.
You enter the amount that the US company has charged you as the net and select the VAT rate that you would have been charged by a UK supplier selling you the same service (presumably 20%). This will add on that amount of VAT at the line level but then deduct it again at the bottom of the invoice, leaving the total equal to the net you were actually charged.
On the VAT return this will show the VAT in box 1 (sales) and box 4 (purchases), so the net result is zero.