Fly doesn’t have a managed Postgres database (yet). If you’re referring to Fly Postgres, you’ll have to generate a custom image where you install the extensions you need.
If you’re referring to the Supabase integration, then it may be possible using their SQL Editor. Their docs say this:
Supabase is pre-configured with over 50 extensions. You can also install your own SQL extensions directly in the database through our SQL editor.