Hopefully the car is not locked and running...
If the piece you have is the cut part, you can take it to the hardware store or anywhere they make keys and try to have another made. Otherwise, you can call your Ford dealer and provide the VIN number and see if they can get the key code from there. Keep in mind that they might want you there in person (bring the broken key) because obviously, this would be an easy way to steal cars - and they don't want to be liable. Once you get a key that works, have 2 or 3 more made for spares. Use a black electrical cable tie to attach one to the frame or bumper support or behind the license plate - wherever you'll remember that you have a spare key hidden somewhere on the vehicle. Good luck.