That is super stressful, but don’t worry! You can open the gas tank on a 2013 Volkswagen Passat by pressing the fuel tank door on the outside of the car.
The reason you can’t find a button inside to open it is because it doesn’t have one.
Once you press on the fuel door, it should pop open and give you access to your gas tank. From there, all you have to do is unscrew the cap and fill ‘er up!
There is one caveat: For this to work, you’ll have to make sure your Passat is unlocked. This is a safety feature that prevents tampering or theft of your fuel.