To get a tax deduction for stock losses, you enter a cost basis higher than the sale price. 

You will still input the information in the Income & Expenses portion of your tax interview. 

Here is how:

Scroll to Investment Income 

Click show more

Select Stocks, Mutual Funds, Bonds, Other

Did you sell any investments in 2018? click "yes"

Follow the remaining instructions

The maximum loss that will be on your tax return is $3,000. Any remaining loss will carryover to next year and will reduce the tax you pay on future capital gains.

For more information, follow this link: IRS on Capital Gains