Replying from my lemmy.ml account since lemmy.world appears to be down but:
some query community:[[email protected]](/c/[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%21ultralight%40lemmy.world&page=1 I was able to at least find 2 posts.
Checkout https://www.search-lemmy.com
I've been talking with the devs of various 3rd party apps to see about getting it added there. I'll eventually reach out to the Lemmy devs themselves once they aren't as busy.