NAT-config, only local LAN IP is showed. How to use the private tailscaled IP ?

In the NAT configuration, the local LAN IP is showed. OK.
But the server is also on a tailscale network (tailscale.com),
and the IP private ip 100.***.***.*** of the tailscale network is *not* showed in the select IP for the NAT-configuration.
Question : where can we modify the IP's that are showed in the HTML-SELECT with the local IP ?
Is there somewhere a script or config-file we can adapt for that ?
Thanks