Create a systemd service for Softether

sudo nano /lib/systemd/system/vpnserver.service
[Unit]
Description=SoftEther VPN Server
After=network.target

[Service]
Type=forking
ExecStart=/usr/local/vpnserver/vpnserver start
ExecStop=/usr/local/vpnserver/vpnserver stop

[Install]
WantedBy=multi-user.target
sudo systemctl enable vpnserver.service

References
https://linuxconfig.org/setting-up-softether-vpn-server-on-ubuntu-16-04-xenial-xerus-linux