You have to be careful that you're in the right spot, which is the site-wide search bar instead of the magazines search, and you don't need the "!" prefix.
Visual guide: https://kbin.social/m/quickstart/p/568882/How-to-add-a-community-from-the-Fediverse-if-it
Post with other tips: https://kbin.social/m/RedditMigration/t/100810/New-kbin-user-guide-preparation-for-impending-wave-of-reddit