It looks like in the past it was possible to bring up an arbirtrary interface (i.e. tun) and then it would be configurable in the GUI (see https://forum.opnsense.org/index.php?topic=3168.msg10301#msg10301 ), but this doesn't seem to work anymore. I'm working with a tun interface that works perfectly fine on the BSD layer but for some functionality I need to add it to the OPNsense configuration as well.
Any suggestions how to achieve this?
opnsense will filter interface name in the webui, you can use tun name like ovpns999 or ovpnc999
The "ovpns" and "ovpnc" trick will go away in 22.1.2 permanently after a brief time in 22.1.1 already.
You can use any unused device name really, I would recommend "custom" prefix or something that will never clash with existing VPN software or FreeBSD drivers. Just rename the device properly after creation (or better yet while creating it).
Cheers,
Franco