Hello,
UDP Broadcast Relay and RelayD packages are available from the package manager in Services -> Package manager. The settings for these packages will become available in Network -> Interfaces -> Relayd / UDP Broadcast relay.
Keep in mind that UDP broadcasts do not work with layer 3 VPNs. Hence, a layer 2 VPN is needed. For example, OpenVPN in TAP mode.
In case you do not see the VPN TAP interface in the relay settings, you will likely need to add the interface (tap0) from the command line by editing the config file:
OpenVPN TAP mode configuration examples are available here and here.
Kind Regards,
Andzej