Nowhere. The payments are not a directly deduction.
If your truck is used for business, go through the vehicle section of the business area. It will give you the option to use the Standard Mileage Rate or the Actual Expenses.
If you use the Standard Mileage Rate, that has almost everything built-in to it, but it will also ask for the interest that you paid for your vehicle (which is part of your monthly payment).
If you use the Actual Expenses, it will ask for the interest, and it will also ask you for the cost of the vehicle so it will depreciate the purchase price. That is a sort-of a deduction for the principal part of the monthly car payment.
