Amending my 2019 returns already Efiled and accepted by IRS Forgot to apply the overpaid tax amount of 2018 towards 2019 return How to apply for refund of underpayment penalty

Wait until the IRS fully processes your return as this is an error they usually find and correct for you automatically.  They will send you a letter when they have made the correction.  If you have a balance due to pay even after the overpayment has been taken into consideration then make sure you pay it timely.