If you have already filed your return and it has been accepted by the IRS, then you will need to file an amended return (Form 1040X) to add your capital loss carryover to your return. If your return has not been accepted, do not make any changes to your return until it has either been accepted or rejected.
Plus note that amended returns need to be printed and mailed, as they are not allowed to be e-filed.
Below are steps to amend a 2018 return:
After you start your amended return, below are steps to be brought to the section of TurboTax to enter a capital loss carryover: