1. You can pay it with the extension and then apply it to 2020 estimated tax when the 2019 return is completed.

2. Have client pay at irs.gov

"Re: WHEN FILING ADDITION EXTENSION TO OCT 15 FOR A PERSONAL RETURN, HOW DO I MAKE 7/15 ESTIMATED PAYMENT ELECTRONICALLY"

Make the additional payment with the extension, then apply the 2019 refund to 2020 on the actual return when you file it.

What if I already filed the extension but now my client wants me to electronically make a payment for 2019?