You need to push the IPv6 /64 as a route. It needs to be distinct from the tunnel network. I assume you have more than a /64 to use? /48 or /56?
Similar to how HE's TunnelBroker provides IPs, Unfortunately TunnelBroker does not work in this case because they Block CloudFlare (YES THEY FREAKING BLOCK CLOUDFLARE!!!).
Based on my experiences with HE over the years, if they did in fact block these sources, they have a good reason for doing so.