Your Horizon Hopper uses Diesel.
To fill up, open the hatch right next to the driver’s side door:
Your Horizon Hopper runs on Diesel and thus needs DEF (Diesel Exhaust Fluid).
What is DEF (Diesel Exhaust Fluid) and why do I need to refill it?
If you are running low, it is shown in the cockpit. Refill then under the hood - on the left side: