This article has the different ways to install Webmin (note: this only contains Webmin, not Virtualmin).

YUM (RHEL-based)

(echo "[Webmin]
name=Webmin Distribution Neutral
baseurl=http://download.webmin.com/download/yum
enabled=1
gpgcheck=1
gpgkey=http://www.webmin.com/jcameron-key.asc" >/etc/yum.repos.d/webmin.repo;
yum -y install webmin)

APT (Debian/Ubuntu)

On Debian, make sure wget is installed first.

sudo sh -c 'echo "deb http://ftp.au.debian.org/debian/ buster main non-free" > /etc/apt/sources.list.d/nonfree.list'
sudo apt update
sudo apt install wget

Once wget is installed or on Ubuntu/Mint, you can then run the following:

wget -qO - http://www.webmin.com/jcameron-key.asc | sudo apt-key add -
sudo sh -c 'echo "deb http://download.webmin.com/download/repository sarge contrib" > /etc/apt/sources.list.d/webmin.list'
sudo apt update
sudo apt install webmin

Post installation

Make sure port 10000 is opened in your system firewall and then navigate to https://<your-hostname-or-IP>:10000 and then login with your server's root user/pass and complete the setup.