I wish I could, I never found any documentation on how to send the 2FA token to the server using OpenVPN connect. Only the windows client community edition is clearly capable of doing it (not tested personally, but many testimonials here of its success). I tried with IOS and android, never managed to do it. As well as googling to the death the web, I could find nothing.
I always thought it was a problem of OpenVPN connect, but probably it is instead the way OpenVPN server is implemented in IPFire the reason of this failure (see thread below).
In this thread we discussed the problem with someone that contrary to me knows what he is talking about.
I wanted to open a bug report, but I never found the energy and time to do it.
EDIT: 13097 – OpenVPN, 2FA infrastructure is not working using ANY client EXCEPT windows community edition