Replying from my lemmy.ml account since lemmy.world appears to be down but:

some query community:[email protected] would search your community. Now, if this community is less than 24hrs old, it may not have been indexed yet, so you may just need to wait a day or so.

I just tested with https://www.search-lemmy.com/results?query=test+community%3A!ultralight%40lemmy.world&page=1 I was able to at least find 2 posts.