In case that was not clear it should go like this.
Go to assign interface: Assign an interface to OPT1 then save
Go to Interfaces:OPT1 Now enable it with checkbox.
Set as "IP4 Configuration Type" as : Static
Now scroll down and setup your new OPT network with a new subnet of your choosing.
Under :"Static IPv4 configuration"
Perhaps something like 192.168.10.1 then >> "/24" << from dropdown, Then save.
Next go to Services/DHCP Server and enable an DHCP server for your new OPT1 address. (Tab at top for new OPT1 interface)
Range in example would be 192.168.10.100 for start and 192.168.10.200 for end of address range. Or similar…
Now go to GUI menu and setup Firewall Rules to allow traffic: Firewall/Rules Go to tab at top for your new OPT1 interface.
Create a rule to suit your traffic. Allow any to setup and perhaps lock down once you get it running.
At this point you can test your interface with a client and ensure getting an IP address and can surf the web.
If all works good than follow the Captive Portal doc's for your new interface to allow traffic.