Then this has nothing to do with your 2018 taxes. However, if you withdrew a large amount in 2019 and it was an early withdrawal, the plan administrator has at a minimum (as required by law with a 401(k)) withheld 10% for taxes and if an early withdrawal, an additional 10% for the penalty. So figure the penalty is paid.
Then look at your tax bracket for 2018. If you're in the 22% bracket for example, then 10% is already paid and you can go ahead and send the IRS an additional 12% if it makes you feel more comfortable on that front. You can even pay online at
www.irs.gov/payments. Just make it a 1st quarter 1040-ES payment and you're good. Remember to print your receipt when done. As always, when dealing with the IRS don't forget the three golden rules:
1) You are guilty until proven innocent.
2) The burden of proof is on the accused (that would be you!) and not the accuser.
3) If it's not in writing, then it flat out, no way, no how, did not occur.
Finally, if your state taxes personal income then you may need to pay additional tax to your state too. How you do that depends on your state. But you should be able to find out at your state's department of taxation website.
