Change default search engine

[yellowhat]

Hi,
I would like to change the default search engine via the configuration files.

I am able to modify other settings via ~/.var/app/io.gitlab.librewolf-community/.librewolf/librewolf.overrides.cfg

But even if I add:

defaultPref("browser.policies.runOncePerModification.setDefaultSearchEngine", "searxng");

The settings is not picked up:

If I understand correctly that setting should be set in the policies.json but I cannot figure out where this file should be placed when using flatpak.

Any suggestions?

Thanks