How do I start a 1120S form on Turbotax self-employed online?

You can't do that - Must use TT Business, an entirely different program, which must be purchased and installed in Windows.

There is no Online version of the software that prepares an 1120S return.   You need TurboTax Business for the S-Corp 1120S return.   You can still use the online version for your own personal return though.

You can purchase the Business program here:
https://turbotax.intuit.com/small-business-taxes/