Ox.el contains the function org-export-define-backend. One of its
keyword arguments is :menu-entry. The examples given include
(https://code.orgmode.org/bzg/org-mode/src/master/lisp/ox.el#L1214)

    '(?l "Export to LaTeX"
         (?p "As PDF file" org-latex-export-to-pdf)
         (?o "As PDF file and open"
             (lambda (a s v b)
               (if a (org-latex-export-to-pdf t s v b)
                 (org-open-file
                  (org-latex-export-to-pdf nil s v b))))))

This is invalid for two reasons:

1. The ?p and ?o elements should be wrapped in an extra layer of
parentheses. For example, the ?p element should be ((?p "As PDF file"
org-latex-export-to-pdf)).
2. There is an extra parenthesis at the end of the block.

I cannot submit a patch, as my employer currently will not sign the
copyright assignment. I hope someone else picks this up and fixes it.

------------------------------------------------------------------------


Emacs  : GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.20, cairo version 1.16.0)
 of 2021-07-09
Package: Org mode version 9.4.4 (release_9.4.4 @
/usr/local/share/emacs/28.0.50/lisp/org/)