It's on the Earned Income Credit Worksheet.  Below line 5 there is a smart worksheet that allows you to elect to use 2019 earned income.  Don't forget to also make the election on Schedule 8812 if necessary.  That election is to use the 2019 earned income to calculated the additional child tax credit.  That option is found in a smart worksheet on Schedule 8812 under line number 5.  You have to elect each one separately if your client is eligible and it is advantageous to do so.