If you've only got 1 key you'll need professional help to get a new key cut and programmed to your car, from Ford or an independent car security specialist.
Independent might be a bit cheaper but £100 is the ball-park you're in assuming you want one for remote locking and your car has that facility.

Overall Ford is probably the safest route to be certain it all works properly but maybe not the cheapest.


Just note that some (cheaper) suppliers may "clone" the existing immobiliser chip, that's not really ideal. Best to have a new unique one and get the car properly programmed which is what Ford "should" do.