Welcome! There's no starting kilometers to add. Next time you fuel-up your Gilera Runner enter you km for that fuel-up at that time. If you're not sure how far you've driven, reset your tripometer and add your km at your next fuel-up.
If you don't have a tripometer or don't want to track it yourself, you can switch to odometer tracking and enter your odometer reading each time you add a fuel-up.