You will enter this in the Vehicle expenses section of your business return. Click on Income & Expenses, and click "Start/Review" next to Self-employment income and expenses. On the next screen click "Start/Review" next to the business name. On the next screen "Here's your Business info", scroll down to Vehicle and click "Start/Edit" (if you don't see Vehicle in the list, click "Add expenses for this work" and scroll further down the list.). On the next few screens, you'll be entering information about the vehicle. If you want to elect the Section 179 depreciation, you'll need to tell the program you will be deducting "actual expenses" rather than the standard mileage rate.
You'll eventually reach the screen where you select the Section 179 deduction where you may also select the special depreciation allowance. (see screenshot below). Follow the remaining program prompts to complete your entry.
