How do I specify that the deceased taxpayer's refund goes to the account of the executor?

[sibbis]

TurboTax specifies that the refund must be deposited via direct deposit to the taxpayer's bank account. The taxpayer no longer has a bank account because he died last year.

[M-MTax]

You need to file Form 1310.

https://www.irs.gov/pub/irs-pdf/f1310.pdf

[JamesG1]

For a deceased taxpayer, IRS form 1310 should be completed.

This TurboTax Help states:

If a refund is due, there's one more step. You should also complete and file with the final return a copy of IRS form 1310, Statement of Person Claiming Refund Due a Deceased Taxpayer. Although the IRS says you don't have to file IRS Form 1310 if you are a surviving spouse filing a joint return, you probably should file the form anyway to head off possible delays.

In TurboTax Online, enter 'form 1310' in the search microscope in the upper right hand corner of the screen.  Select 'Jump to form 1310'.

@sibbis