They took a photo of a broken car key, processed it in Fusion 360 and 3d printed in on Object Connex 500. You could use software you are used to or use Shapeways for printing the key. Simple FDM home printers were not used in the process so I'm not sure how the end result would be.
Do keep in mind that some keys have security codes so maybe you will not be able to start your car or you will trigger some sort of security protocol.
You can see a full tutorial here:
If you want to do this yourself, you will have to watch out for the thickness and strength of your 3d printed car key and that you don't brake it inside your ignition lock.
And now stories will be posted by fearmongers on vehicle security and potential for crime and how you should always keep your car key out of sight so that they can not be copied by taking a photo of them ...