Not sure what Schedule D would have to do with a license plate discount. Schedule D is used when you enter such information as capital gains on your tax return. If you have a Schedule D it will be with the other forms and schedules that are part of the pdf of your tax return.
Here is some information on Schedule D and what it looks like:
https://www.irs.gov/forms-pubs/about-schedule-d-form-104
https://www.irs.gov/pub/irs-pdf/f1040sd.pdf
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