How to add fw4 custom script?

How to add fw4 custom script using iptables?

I tried to add an include config in /etc/config/firewall but it is not called by running fw4 reload or /etc/init.d/firewall restart.

In older version of OpenWRT there was a /etc/firewall.user script, but I cannot find it in snapshot.

Is it a feature or a bug? If so, how can I add iptables scripts to fw4? (currently I have difficulties to rewrite all the iptables rules to nftables)

Converting iptables to nftables is required and easy, share your script if you want help.