My app integrates with iOS and Android calendar API's and I include links in the notes. Currently I just use long URL's and the iOS app for example links them. However, they are unsightly to the user. It would be better to have linked text i.e. HTML such as "Open in XYZ App" which is linked.
Do calendars such as described above support HTML in notes as well as Outlook for example?
As of today the iCalendar format (RFC 5545), and hence CalDAV based services, do not support rich text notes on events. Since this is the base technology for the iOS and OSX calendar applications / EventKit (and many others), those do not support them either.
As you mention you can link to web pages via the URL property (and OSX/iOS data detectors might detect URLs in plain text links). If you click such a link, the regular link flow happens (and will open proper apps).
But maybe I misunderstood your question, i.e. what you want with that "Open in XYZ App" thing.