You have 2 options.

You can do as said above, have a key added at a dealer or independent mechanic with correct type OBD II tool (not just a code reader) and have them program another key to the system (key needs to be one with RFID chip and already cut), or you can use the Forscan software with correct OBD II adapter if you want to risk it.
Once you have 2 keys, you can add more keys yourself (up to 8 I believe).
My mechanic has the correct OBD II tool to do it with. I bought 3 keys on e-bay for like $10 and had them cut at a local hardware store for $6. My mechanic charged me $40 (1/2 hr labor, which is how long it takes to do it) to program 2 keys (original key and 1 new one).
So less than $60 and I have 4 keys (2 of which I programmed myself)

The other option is to go to Home Depot, Walmart, Lowes, or most any locksmith and have a key cloned. This just creates an exact copy of the old RFID on the new key. So while you will have 2 physical keys, the truck computer sees them as the same key. You can't add more keys yourself with a cloned key since you must have 2 unique RFID keys to do it.
Charge for a cloned key is ~$50 when I looked a few years ago.

 

New member needing help. I planned on purchasing a 1999 F150 Lariat with NO key. Ford will not order me a key because the truck is too old. Will this program work for this situation or is it possible to change out the control box (whatever that is) & use a different key?

 

Dont.

 

Any special reason for your answer? No to the truck or the program or everything? I am unsure what controls the program for the key.

 

If the truck doesn't have a key how can you assess the condition of it mechanically? As far as I'm concerned in that condition it's value is scrap. You will need to tow the vehicle to the dealership and they can program a key for you. This is the problem of whoever wants to sell the truck..... Not who wants to buy it.

 

I was seeking advice on what I found was the anti theft system. I was hoping to find it feasable to make a fix. This is more complkex than I thought & I passed on the purchase. I learned it is fixable but not worth it. Being a 1999 makes matters worse. Thanks for the help.

 

It is EASY to reprogram PATS these days using Forscan (with extended license) and an appropriate OBDII adapter.

All you have to do is buy a new lockset with two keys, replace the cylinders, and go for it.