On Tue, Jul 19, 2022, 10:54 AM Uwe Brauer <oub@mat.ucm.es> wrote:

>
>
> #+begin_src elisp :noexport
> (defun my-set-property-at-heading ()
>   "Function to be called at the beginning of an Org heading."
>   (interactive)
>   (let ((el (org-element-at-point)))
>     (org-set-property "New" (org-element-property "[ ]" el)))))
> (org-map-entries #'my-set-property-at-heading)
>
> #+end_src
>

Try replacing the `org-set-property` form to:

(org-set-property "New" "[]")

[Not tested, typing from phone.]