We had a bit of an investigation and unfortunately there’s no easy answer.
First, on iOS12 the reminders don’t have a URL field visible in the Reminders.app UI, so even if we insert an Agenda URL field there, it wouldn’t show on iOS12 (nor in macOS 10.14 where they currently do work). In the notes field (like we do now) it is visible, but unfortunately Apple doesn’t support clickable links on iOS12 in the notes field (unlike on Mac 10.14 where they do work).
On iOS13 Apple supports the URL field and has a fancy look for links with an icon (though in my testing on Mac clicking them doesn’t seem to open the link, not even for www.apple.com, probably broken in the current beta). Unfortunately it seems URLs in notes don’t work here either, and not even on Mac anymore, not sure if that’s broken too or no longer will work.
This leaves us some nasty choices, moving to the URL field means no agenda links on iOS12 and macOS14.5 Leaving them in the notes field means no clickable Agenda URLs possibly even on mac. Putting them in both could be a solution but is quite ugly of course. We’ll continue testing and then will decide what to do. Stay tuned.