Hello @smoreland1,
As of the moment, there isn't an integrated way to withhold an additional 20% federal taxes for your employee. This is the reason why you received such an error prompt when setting it up in the system.
That being said, you can consider manually calculating the 20% withholding amount of your employee's taxes and enter it directly on the Extra Withholding field under payroll information of your employee's profile.
Additionally, you can also browse for the content of this helpful article to get a better view of how your employee's taxes are calculated: Understand the tax amounts reported on your employee's paycheck.
If there's anything else that I can help you with, please let me know in the comments. I'll be here to lend a hand. Stay safe!