Wireguard - Trouble setting up IPv6 for external VPN

[FormBurden]

After tons of trial and error, I have finally got Wireguard to work with Mullvad VPN using the Wireguard app in OPNsense. IPv4 works great, but the part in all the config that I'm stuck at for IPv6 is the gateway. In a lot of tutorials for Wireguard in OPNsense for external VPN, you set up the gateway on the config of Wireguard to be something similar to one number down on a IPv4 address (eg: 1.2.3.4 > 1.2.3.3) by using this command:

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curl -sSL https://api.mullvad.net/wg/ -d account=123 --data-urlencode pubkey=PUBKEY
But how do I do all of this for IPv6 address? I get the IPv6 address from the above terminal command. So it's inputted in the "local" on Wireguard for the Tunnel Address.

IPv6 seems hard to grasp because the one down method for IPv4 doesn't seem to work if trying to attempt the same principle for IPv6, or I just don't know how to "one down" a IPv6 address. I just get those endless errors of "does not lie within one of the chosen interface's IPv6 subnets"

So how would I do this for IPv6 address?

Also I would assume the firewall rules for IPv6 are pretty much the same as the IPv4 addresses for IPv6?

All the info I could find for Wireguard on OPNsense are all for IPv4, never really going into detail on how to do it for IPv6 (such as how to do the gateway for IPv6).