You will not need to to actually access the Schedule 8 to make the election to stop contributing to CPP in TurboTax.
To make the election (or enter the details of a previous election to stop paying CPP on your Self-Employed earnings) using TurboTax Online or in the EasyStep view, go to Income > T-Slips > T4A(P) Canada Pension Plan Benefits.
On the T4A(P) - Summary screen, you can enter your information about previous elections or choose to stop contributing to CPP on your self-employment earnings. You must say "Yes" to the question - "Do you wish to stop contributing to the Canada Pension Plan (CPP) on your self-employment earnings?"
This screen will only appear if you were 65 to 70 years of age.