I’m not understanding how you ended up so badly under withheld. Make sure you use the IRS estimator to confirm this. You may also want to use the TurboTax tax caster for another forecast and see if they are similar.
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
yes, you can make an additional payment now at the IRS website, www.irs.gov/payments. Make sure you select 2020 estimated payment as the reason. If your estimate is high or low, the difference will either be refunded to you or result in an additional payment owed when you file your tax return.
However, you will still likely be assessed a penalty for being under withheld during the year. The tax system is supposed to be pay as you go, the IRS expects that your tax payments will be made in roughly equal installments. If you are under withheld during the year, you can be assessed a penalty even if you pay in full by the time you file. If this is your first time owing a penalty, when you get the notice, you can apply for first time penalty abatement. Also note that any interest you owe can’t be abated under law, but the interest will be recalculated if the penalty is reduced.
https://www.irs.gov/businesses/small-businesses-self-employed/penalty-relief-due-to-first-time-penal...
