An extension allows you to delay the filing of your tax returns. The due date is April 15, 2019.
If you have already filed your tax return and want to add your car, you will file an amended tax return.
Since it appears you are self-employed, you can deduct the expenses for your car to the extent it was used for your business.
For more information, follow these link:
What to Include with Amended Tax Returns
How to Amend
Tips to Amend
IRS on 1040X
