D7: Enter an Amount for extra Federal Tax Withholding
If you know an approximate or exact actual dollar amount you wish to withhold in addition to your current W-4 based withholding amount, enter that amount here. Unfortunately, the W-4 does not allow you to enter a total fixed tax withholding dollar amount per pay period.
Since the W-4 is a planning tool, you can increase your pay period tax withholding by entering an amount here and see on the right side how your tax withholding would change.
Alternatively, you could make one or more estimated tax payments on the IRS site instead of withholding additional taxes here.
Your Federal Tax Withholding Calculated Amount is: