The bucket needs to be set to public, either via UpdateBucket() or by going to Storage on supabase.com/dashboard, clicking the overflow menu on a bucket and choosing "Make public"
Policy permissions required:
buckets permissions: none
objects permissions: none
Examples
Returns the URL for an asset in a public bucket
var publicUrl = supabase.Storage.From("avatars").GetPublicUrl("public/fancy-avatar.png");