doktornotor pointed to the method how to set it up with HAproxy whenthereisn'tawebserveronport80*
HOWEVER: The default nginx Webconfigurator, will also listen on port 80 when the "WebGUI redirect" is unchecked (System -> Advanced -> Admin Access)
Then, under the certificate under the Services -> ACME, select/edit/create the certificate, you select the webroot local, and then use /usr/local/www/.well-known/acme-challenge/
(See attachment)
I suspect when I check that WebGUI redirect disable, then you could use the "standalone HTTP server" option…

