This method requires a little bit of patience but isn't too hard. After spawning inside the Ultor lobby, you'll want to open both sets of doors that lead inside. Once that is done, stand in front of the building and wait for cars to pass by until you see a Comet, which is a small white hatchback. Once it does, quickly hijack it! The sprinting hijack is probably recommended, and once you're inside, quickly turn into the lobby and make the car hit the main desk inside the building.
It is NOT recommended that you "Initiate Balls of Fire" once you get inside the car or else it will be significantly harder. Also, if you move too far from the doors, they will close, so do not leave the area. You can try to get in the car, drive it over, park it, open the doors and get back in, but there is a good chance that the car will get damaged and begin to explode. Once it is on fire, you will not be able to get back inside which is what makes it problematic.Co-op Method:
Do the same method as above, but you can go ANYWHERE to find the car. Once you have it, drive it back in any condition and then have your co-op friend open the doors for you!