Need help for luci (cbi, uhttpd)

Hi, I want to enable/add an option ubus_cors to default in /etc/config/uhttpd when flashing new firmware which was not there before. What script do we need to modify for this change ?

I found lines related this in uhttpd.lua as mentioned below:

o = ucs:taboption("advanced", Flag, "ubus_cors", translate("Enable JSON-RPC Cross-Origin Resource Support"))
o.default = o.disabled
o.optional = true

i have a question also

what does o.default = o.disabled and o.optional= true actually do ?