How do I send the IRS witholding taxes on a Roth conversion?

[VolvoGirl]

Why do you need to send it?  Didn't the IRA plan take it out for you?  If you need to send in an amount to cover the tax you send in an Estimated payment.

Here are the blank Estimates and instructions…..
https://www.irs.gov/pub/irs-pdf/f1040es.pdf

Or you can pay on the IRS website. Be sure to pick 2021 1040ES payment
https://www.irs.gov/payments

[Critter-3]

Conversions usually don't have withholding from them as it would defeat the purpose of the conversion ... sending in an estimated payment is the way to go ... you can pay online ... see the other answer for the details.