The command I posted above is only an example! You need a configuration file generated by protonvpn to open up with openvpn.
You need to login to protonvpn web site with your account. You must navigate to the page listing the free servers. Choose one and download it's configuration file. This is generated by selecting from some options that fit your situation and a list of servers is created. Select one and click the download button. The login credentials are on your Protonvpn Account page near the bottom in the Manual login credentials.
The configuration file you see in the example command is a symlink to a file us-free-158078.protonvpn.udp.ovpn, which is generated on my account page on the protonvpn web site.
