Hi,
it’s a bit confusing for newcomers. The search box is not intended for IDs (we are about to change that)
So you would have to go the way using New Entry → And then import by ID.
But this is cumbersome.
But the easiest option for batch would be to do a search on the pubmed website, and then
Send to → Bibliography manager → Create file
And then you can import the nbib file in JabRef via File → Import
A third option is to use the JabRef Browser extension. But that will only work for visible citations on a page.