There is no Signal-FOSS fork from F-Droid currently.
You can add the Signal-FOSS repository to F-Droid to get the App and updates via F-Droid … https://www.twinhelix.com/apps/signal-foss/ … that’s different.
It doesn’t seem like App Lounge can handle this yet …
… but I don’t use it currently, so I don’t know the current status.