You need to put the full url of the sublemmy you are looking for into the search box.
That will then cache it against the instance and we will all then see it in search.
Content from the sublemmy may take some time to load after, potentially a few hours.
If you don’t know the sub you are looking for, google search it to try and get its url from there.
This is documented in the Lemmy manual, I had to do it myself