Wake On Lan broadcast across VLAN

Hi

I have a Fortigat 200 (v7.6.6)

I have a server WAPT on vlan 10 and I want use it to wake on lan my PC in vlan 20.

Unicast WOL is working 

wakeonlan -i <ip> <mac>

But not with broadcast WOL. And WAPT use only broadcast mode for doing that.
When I try to wol aPC (10.20.20.1) from wapt server (10.10.10.1) on vlan 10 :

# diagnose sniffer packet v10 'dst 10.20.20.255' interfaces=[v10] filters=[dst 10.20.20.255] 3.495703 10.10.10.1.41831 -> 10.20.20.255.7: udp 102 3.495938 10.10.10.1.48121 -> 10.20.20.255.9: udp 102 # diagnose sniffer packet v20 'udp' interfaces=[v20] filters=[udp]

I have taken a look here

https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-to-route-Wake-On-Lan-WOL-magic-packet-through/ta-p/198103

but I don't understand if I need to add an entry in arp table for each PC I want to wake up (I have 2000 PC).

Or if i need to enable this command 

config system interface   edit <external_interface_name>   set broadcast-forward enable end

But what is "external_interface_name" ?

Thanks for your help