Route another subnet through ipsec tunnel

[supportadl]

Hello All.

I'm new to this forum and pfsense too.
I would like your help configuring a routing rule on my running pfsense firewall.

My configuration is

Site A (headquarter)
Firewall Zywall USG 110
Lan 192.168.1.0/24

Site B (cloud rdp server)
Firewall Pfsense
Lan 192.168.40.0/24

Site C (remote office)
Firewall Zywall USG20
Lan 192.168.4.0/24

There are 2 ipsec tunnel running correctly.
Ipsec Site A to Site B
Ipsec Site A to Site C

I would like to reach site B from site C (and site C from site B) without configure a new ipsec vpn between them because in the future I will have more remote office.

I already created some routing policies on Zywall for site A and C but I don't know how to tell Pfsense that traffic from 192.168.40.0/24 to 192.168.4.0/24 must go out trough Ipsec tunnel.

Can anyone help me please?
Thank you very much.