1. Go to the screen that reports that particular business' income;
2. Click on the Less Common Scenario tab;
3. Scroll down to the Prior Year Unallowed Passive Losses section; and
4. Enter the relevant details under QBI.

The other answer explained what to do if the QBI carryforward was from passive income, but there can also be QBI carryforward because the business had a NOL the previous year. 

In that case, it goes on Income -> Net Operating Loss -> Federal Net Operating Loss -> Qualified Business Loss Carryovers -> Qualified business loss carryover. 

For 1040 look at Net Operating Loss > Federal Net Operating Loss and scroll to the last set of 3 entries.