How are y’all making this so complicated!?!? 
Just contact Webmin and Usermin on a domain name that has a working cert. Don’t fiddle with it. Just use it the way it was intended to work…i.e. if you’re hosting example.tld on your server, and it has a working TLS certificate, then browse to https://example.tld:10000. It will use the cert for that domain.