mcedit /etc/apt/sources.list

...
#webmin
deb http://download.webmin.com/download/repository sarge contrib

apt update
wget -q -O- http://www.webmin.com/jcameron-key.asc | sudo apt-key add
apt update
cp /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d
apt update
apt install webmin
ls -l /usr/lib/systemd/system | grep service
ls -l /usr/lib/systemd/system | grep service | grep webmin
systemctl enable webmin.service
systemctl start webmin.service