The deposits into the 401(k) must be made by the S corp and reported on the S corp's tax return. The S corp takes the deduction. The S corp will report in box 12 of your W-2 with code D for traditional elective deferrals or code AA for designated Roth contributions. When preparing your tax return, you will enter the W-2 and enter amounts in box 12 of the W-2 into box 12 of TurboTax's W-2 form, nowhere else in TurboTax.
The link provided by MinhT1 only applies to individuals who are self employed and you are not self-employed (except for the purpose of a self-employed health insurance deduction). You are an employee of the S corp.
