How can I set a flatpak app to start with a certain portal by default.

Some flatpak apps, such as Discord in this case, don't have the proper portals setup, meaning I can't access most files and folders. I can easily fix that by launching it from the terminal with option "--filesystem=host", but that means I need to keep launching it from the terminal if I want that portal working. So how can I set it to launch with the "--filesystem=host" portal by default, such as if I launched it from the Gnome Activities menu? Am I missing something obvious in the flatpak documentation, like usual?

I've been playing around with Fedora Silverblue on a spare hard drive, and have been learning a lot about flatpak as a result.