The fuel door release lever can be found on the floor of the driver’s seat in your Forester. It will have a little gas pump icon on it and should be pretty obvious once you know what you’re looking for. When you pull up on the lever, you should hear a small pop as the fuel door on the outside of your car springs open.
Finally, get out of your Forester and swing the fuel door all the way open. Then, unscrew the gas cap underneath and you’ll be able to fill up without any problem! Just make sure to replace the gas cap and push the fuel door back into place when you’re done.