Enable FlatpakOpen Discover and click on Settings (lower left corner).
Check Flatpak settings
Check that in the Flatpak section the box is checked.
Note: with this Flathub app search will be integrated in Discover, if you want to limit the app search to Flathub you can mark Flatpak as default by clicking on the star.
Flatpak support is built into KDE neon 19 and newer—no setup required!
If you are using an older version, you can refer to the instructions below.