Model/CBI -- how to get values in TypedSection

I want to create a luci app for the https_dns_proxy package (I've also made the init script more OpenWrt-like and will be submitting PR pending Aaron's review) and I want to conceal some settings from the user and essentially give them the drop-down for picking provider rather than needing to input specific details:

The only issue is setting the prov.default -- how can I obtain the value of url_prefix from the same section?

You cannot reference specific uci values at declaration time, at least not for typed sections where the actual ID is not known in advance. But maybe I misunderstood what you try to achieve.