Compose key with FCITX

I'm using Arch+KDE Plasma. In the Plasma settings, I added the Compose-key to right-control. Didn't change any other settings. I recently installed FCITX (with the French AZERTY and the Japanese MOZC keyboards). Since then, my right-control key doesn't do anything anymore. I would like right-control to be the compose key with FCITX. Their issue tracker says it has been solved in 2014 , however it doesn't explain how to actually do it. Since the repo is archived, I don't know what's the correct place to ask them directly.