Seems to be lacking, I tried searching for "defcon 28 talks" since it has a hacker news filter and it didn't return anything relevant when compared to my SearXNG search that came back with much better results. I tried it without the hacker news filter and that just made the results worse.
On the web ui, a popover should appear as soon as you type @ followed by three nonspace characters. You can select the user you want to tag from there and it should fill in the proper markdown. In the end it looks like this: @peregus@lemmy.world