DKIM and external DNS servers

I want to enable DKIM for my mail send through a webform, but use an external DNS server.
I followed the setup procedure and added my domain to the ‘Additional domains to sign for’ box.
Howe ver, there is supposed to be a field (after saving I expect) which has the public key, called “DNS records for additional domains”.
This does not appear on that page for me? Where can I find it?

I am running Ubuntu Linux 16.04 (from a distilled-upgrade a while back if that makes a difference) and Webmin version 1.831 and Virtualmin version 5.0 and Authentic Theme 18.40.

If I can find it on the file system then that is fine too!

You do realize that any DNS changes you make in Virtualmin will have no effect on your domains right?

If you are using an external DNS service, then you need to make the changes on that service/server. The changes you make on Virtualmin only apply if you are using the local DNS in your Virtualmin server.