I have filed an extension for 2023, but need to pay the tax liability shown on the extension. What form do I use to accompany the payment.

1040-v  if you mail be sure to use a tracking method so you know its gets to its destination. make sure its postmarked on or before 4/15. Postage meter dates are not respected by the IRS and dropping in a mailbox may result in a postmark later than 4/15. 

or to skip the mailing use the iRS online portal.

https://www.irs.gov/payments 

Thanks Mike.  I think you are correct about the 1040V.  Did not see that I could print that from Turbo Tax.  I also thought this should have been part of the procedure in TT when filing an extension electronically.  I did not want to pay thru the IRS portal, but wanted to send a check.  What I ended up doing was sending in a copy of the extension along with the check, and marked on the check the tax year and social security number.