Hi FrederickZh - currently the Assistant is in limited beta, on a slow release for Ultimate subscribers. All ultimate subscribers are expected to be onboarded likely within the next week, followed by the rest of the Kagi customer base after that.
If you are part of the beta, you should see new icons under the search bar, like above. You will also receive a notification when you are added. After that, the bangs should work.
If you are already part of the beta, let me know and we can look for any additional issues.
