How to add ULA ipv6 subnet

I'm getting ipv6 auto-configured with public addresses using prefix delegation. But it's also convenient to have ULA addresses (in fc00::/7) for local services (for instance they are stable compared to ISP prefix that can change). I don't see an option to configure that in network settings. Thoughts how one can do it? Ideally firewalla would automatically create a ULA prefix for each subnet with a possibility to customize.