Thanks for the reply.
I've called that number multiple times to no avail. They've changed the call tree from the way it was last year (when you could navigate your way through the system, fail to enter some data, and that would trigger an option to talk to an agent).
I may try sending a letter.
I posted about my question on Reddit. Someone there helpfully pointed me to this,
2023 Instruction 1040 (irs.gov)
On page 60 it says,
> Line 36
> Applied to Your 2024
> Estimated Tax
> Enter on line 36 the amount, if any, of
the overpayment on line 34 you want applied to your 2024 estimated tax.
> We will apply this amount to your account unless you include a statement
> requesting us to apply it to your spouse's account. Include your spouse's
> social security number in the statement.
>
> Caution! This election to apply part or all of the amount
> overpaid to your 2024 estimated tax can't be changed later.
