Tiago,
In PFSense, in the DHCP server configuration section, under the section "Additional BOOTP/DHCP Options"
Option, Enter 191
Type, Enter: String
Value, Enter: "pool1=192.168.0.100;level=2" (Include the double-quote marks " Change
the IP address as needed and set the "level=" to either 1 or 2 as needed for your deployment)