The wiki page, linked to, outlines the configuration options that the phone provides. Most of them are for pointing the phone at the proper configuration and cert/key files. One of them, network_default_enable_openvpn is what tells the phone to actually run the OpenVPN client when it’s booting up.
Before the phone’s going to be able to connect, it’s going to need to retrieve the configuration. It’s easiest to use the in-line method. So, create a phone configuration file, hand it the URL for the OpenVPN client configuration (w/ the in-line’d cert/key), watch it load that from the HTTPs URL. Then change the phone’s configuration file to enable OpenVPN and restart the phone.