How to start openvpn or ssh server under Ubuntu 16.04 and 18.04

sudo systemctl start openvpn@client
The "client" is derived from the configuration file name (/etc/openvpn/client.conf). If your configuration file is server.conf, the command needs to be
sudo systemctl start openvpn@server
When you want the service to start up automatically during system boot, you issue:
sudo systemctl enable openvpn@server
Templated versus non-templated services

BSD and Linux

I am used to the fact that a Linux installer honours a pre-existing install of Windows and offers to setup the computer with a dual-boot option during installation. Vice-versa no Windows installer honours any other pre-exisiting OS. Therefore I was surprised that when I tried to install Ubuntu 16.04 on my PFSense box (FreeBSD), the Ubuntu installer did not even see that a BSD install exists on the drive. I chose the "erase all" option, but when I rebooted after the installer has finished, the system went straight into PFSense without giving me any option to select Ubuntu.