There are no startup scripts with systemd, only unit files defining a variety of things, be it a service, a socket, a target, etc. But you can execute a script with such a unit file which comes close to what you mean.
Anyway, it should be possible. Create a basic service file, something like the following, name it like "ovpn-selective.service" or so, then move it to /lib/systemd/system and trigger a daemon-reload::
[Unit]
Description=Selective OpenVPN connection
After=network-online.target
Wants=network-online.target
[Service]
ExecStart=/path/to/the/script.sh --with the --arguments +you -want to +use
[Install]
WantedBy=multi-user.target
$ sudo mv ovpn-selective.service /lib/systemd/system/
$ systemctl daemon-reload
Check that it works. If it does, mark it for startup:
$ systemctl start ovpn-selective.service
$ systemctl enable ovpn-selective.service