Importing website certificates (e.g. NextDNS block screen) in Chromium

Flatpak: Chromium

When I follow the instructions at NextDNS to add a certificate in Linux for Chromium (see [How to install and trust NextDNS Root CA - How-To Guides - NextDNS Help Center]} the certifcate for websites is installed, but when I relaunch Chromium the certificate is lost.

So the obvious reason is that this is not stored permanently in Flatpak Chromium. What folder do I have to give Chromium running in Flatpak permanent write permission to keep this certificate installed?