I'm sure a flatpak wizard will turn up at some point but because it is sandboxed I think you need to launch it with magic.
For example to launch it from the terminal you would need to run flatpak run io.typora.Typora.
If you are also using the Flatpak Joplin then the maintainer has included a useful guide to getting external editors working, I don't know if this will be of any use if you don't have the flatpak version though.
https://github.com/flathub/net.cozic.joplin_desktop#use-external-editor-with-flatpak
