Hello!
This can be done with the virtualmin CLI command.
virtualmin modify-dns --all-domains --add-record “autodiscover A 127.0.0.1”
Replace with the IP you want it pointed to.
Rerun again with autoconfig.
Run the same with AAAA if you also want/need IPv6.
Should do the trick, but you might want to try with one domain first to be sure.
Run ‘virtualmin modify-dns --help’ to see the available options.