That’s actually where I purchased from, clicking on the link that showed the fh4 pre-order and having the Picture of the 720s, but there was nothing definitive about it actually being a pre order bonus.
Go To Festival and get your cars, view all cars then click menu jump to manufacturers and pick cars you got for free. you find them by manufacturer. you should see purchased and new with icon next to it
If you did not get the PORSCHE CODE you will not have the car magically appear in your garage. YOU HAVE TO USE THE CODE TO GET THE CAR. If your game did not come with TWO CODES, one for the car and one for the game, CONTACT the people you bought it from.
After a week of contacting both Microsoft and Amazon UK to chase the code up with the latter dropping me some promo credit for the inconvenience i finally woke up yo my 2nd code via email today so check your emails as they may finally have been sent out after being passed from pillar to post for the last week in my case