If you're using the GUI, manual editing of /etc/config/openvpn, since the GUI does not support all possible keywords. Otherwise, just place it in the config you're feeding to openvpn.
Just like mine, however I have not tried to connect over IPV6 because I don't yet have another tunnel set up to test vpn over IPV6. Plus I am not even sure netstat knows about IPV6 on OpenWRT: I'm sure my router is listening at least on port 80 also on IPV6 but it is not shown. Yup, I have tried connecting with the literal IP in '[...]' notation and it works.
A specific "proto udp6" keyword is needed. Try replacing it in the line above and see if it changes.
EDIT: I'm curious to know if "udp6" implies "udp4" or not, in which case you'd probably need two configs.