When you use the auto-import option, there is a drop-down where you need to choose the previous period CT600 from which you want to capture the data. Alternatively, you may enter the brought forward balance of trading loss by navigating to Corporation Tax >> Tasks >> CT600 >> Edit Return >> Quick Entry >> Trading Professional Profits Calculators >> Trading Losses Brought Forward box.
Please refer below screenshot for more help
Click here to watch how to carry back losses from current period to previous
Click here to watch how to enter brought forward written down values
Click here to watch how to enter brought forward balances of a trading loss