Captive Portal for OpenVPN clients

[elbuit]

Hi.
I'm trying to configure a captive portal for OpenVPN clients.

But configuration options shows WAN as interface to apply but not ovpns1.

I would like to add a custom MFA using the captive portal against a http API.

The first auth factor will be the openvpn client with user/password and the second auth factor will be implemented in the captive portal.

Has anyone done this before?
Thanks.