This isn't currently available on desktop, but if you're using an iPhone or iPad, it will be made clear to you once you get to the checkout page.
When you go to the checkout, ApplePay will be the second option that you're given, so just select "ApplePay" and then continue with your information. Please double and triple check your address if you're paying with ApplePay as you might have a different address saved in there to the one you want your order delivered to.
If you need to add a discount code, then expand the "Show order & apply discount" bar first before selecting the "ApplePay" button (as below).
Please note that your order will be sent to your default ApplePay address, so please ensure that this is correct in ApplePay before placing your order.