Should be a label on it but you may need a mirror and a flashlight to read it.
Or
If you bought new and kept the window sticker it should be on there.
Visit a Ford dealer that is friendly with the vin and explain that you don't want to buy one somewhere else but are trying to rig something on your truck.
Dealers are getting prickly about helping us buy stuff somewheres else. Bribes might work if nothing else does.
Or Fords website may be of help, again your VIN will be needed.