I am trying to change the "Additional Withholding Per Paycheck" for my W-4. However it keeps coming back as zero. How do I change this?

[DawnC0]

In order to get TurboTax to add an  'Additional Withholding' amount, you have to go in and adjust the income or deduction numbers enough to trigger a 2018 tax due.  If you know how much extra you want to be withheld, just fill out the one-page form; it will save you the hassle of trying to get the software to match what you want it to say.

Here is the form if you want to print, complete, sign and return it to your employer:   Form W-4  The 2018 form is not finalized yet, but you can use the 2017 form to give to your employer if you are adding an amount for additional withholding.  Just sign and date it before you give it to your employer.