You can’t. Turbo Tax is only for self prepared returns. Even if you are helping someone it is still self prepared. You can not use the Online versions or the personal Desktop Software to prepare returns for clients. Intuit has 2 products for tax preparers. They are Lacerte and ProSeries. ProSeries is the least expensive. You can install ProSeries and Turbo Tax on the same computer.
Intuit Products for Tax Professionals,
Professional Tax Preparation Software (Online & Desktop) | Intuit Accountants
And to prepare more than five e-files, you will need to sign up with the IRS as an ERO - an Electronic Returns Originator. How to become an Authorized E-file Provider…..
Or do you need to enter a 3rd Party Designee that can talk to the IRS?
You can only enter a 3rd Party Designee in the Desktop installed program using Forms Mode. Not in the Online version. There is no question and answer steps for it. It's on the Federal Information Worksheet, near the end of Part VI, at the bottom of page 2.