Look at the lien-holder's website to see how to send an extra principal payment. Some have the information, some don't. If they don't, try calling for the procedure. With Toyota, we have always sent a separate check to the payment address, with both the envelope and the check labeled "Additional Principal Payment." There is no way to do this on their website. There is a way to send the pay-off amount, because that's not uncommon as cars get paid off or sold.
You have to watch the car loans, as the lenders will occasionally try to apply excess funds to future payments instead of principal.