Cocoa Text System Extension

I started using the Compose Key feature of Cocoa Text System. This is the feature that allows this keyboard behavior in most applications: Option ⌥ + u + u == ü Option ⌥ + - + \> == → Option ⌥ + + 3 == ♥ Option ⌥ + C + C + C + P == ☭ ... etc. It is documented here: https://developer.apple.com/library/archive/documentation/TextFonts/Conceptual/CocoaTextArchitecture/TextSystemArchitecture/ArchitectureOverview.html I would like to extend this to add the 3 Hungarian characters (also capitalized): Option ⌥ + o + o ➔ ő, Option ⌥ + o + i ➔ í, Option ⌥ + o + u ➔ ű Ideally, I want the same UI behavior as with Option ⌥ + u and Option ⌥ + e where you see a preview of the umlaut/accent and then add the desired letter. How is this possible?