to be honest i haven't tested. I gave you the options used by systemd service.
If you want to use the Artix version be sure to remove the fdmove -c 2 1 and exec part . this is done automatically by 66. But i think the one given by @advesperascit correspond to the one given by artix (in better way regarding 66 syntax)

eric Ha yes, the user ovpn is there but if I add again the @runas key I get a permission denied error in opening the certificate and key files in the client folder. I think it's something to do with the openvpn user owning the client folder and requiring a password. Actually I don't even have a server conf file, my VPN just gives a single config file (that should be the client) to put in /etc/openvpn, without need to authenticate. Even simply moving the conf file to /etc/openvpn (outside the client folder) works, using the @advesperascit template.
/etc/66/service/openvpn@:

[main]
@type = classic 
@version = 0.0.1
@description = "OpenVPN daemon"
@user = ( root )

[start]
@execute = (
cd /etc/openvpn
execl-cmdline -s { openvpn ${conf_file} }
)

[environment]
conf_file=!--config @I.conf

Is it fine ? I removed the --nobind option because it's already in the conf file, however I think that this option and --suppress-timestamps are instead needed by the systemd service to avoid conflicts.

Also, I took a look at the s6 service in the Artix repos, it basically uses the same command:

#!/bin/execlineb -P
fdmove -c 2 1
exec openvpn --config /etc/openvpn/openvpn.conf

for the server:

create a file at /etc/66/service/openvpn-server@ with the following

[main]
@type = classic
@version = 0.0.1
@description = "OpenVPN server daemon"
@user = ( root )

[start]
@runas = openvpn
@execute = (
    cd /etc/openvpn/server
    execl-cmdline -s { openvpn ${conf_file} }
)

[environment]
conf_file=!--status /run/openvpn-server/status-@I.log --status-version 2 --suppress-timestamps --config @I.conf

be sure to have an existing configuration file at /etc/openvpn/server/ with the name used to enable the service. For example, if you do 66-enable openvpn-server@myenterprise you need to name the configuration file myenterprise.conf

for the client

create a file at /etc/66/service/openvpn-client@ with the following

[main]
@type = classic
@version = 0.0.1
@description = "OpenVPN client daemon"
@user = ( root )

[start]
@runas = openvpn
@execute = (
    cd /etc/openvpn/client
    execl-cmdline -s { openvpn ${conf_file} }
)

[environment]
conf_file=!--suppress-timestamps --nobind --config @I.conf

Again, be sure to have the corresponding configuration file at /etc/openvpn/client.
like said @advesperascit, no warranties etc

Please tell us if you got it running correctly