I noticed something interesting today, which I feel might be a bug (perhaps on Spark’s side, though.)
I searched for some emails and wanted to select both of them to copy into an OmniFocus task I was adding. I searched, brought up the emails, selected the first one and copy+pasted it into OmniFocus just fine, but when I went to select the second email in the list and copy that one’s link, the Hook app popup showed the same subject line as the first email. I tried double-clicking into the second message to maybe force Hook to see that I’m wanting to copy this second message’s link, and not the first’s, but even then, Hook was still wanting to copy the first email’s link.
I then went and manually located that email by scrolling, etc., found it, clicked it, invoked Hook, and it worked just fine / as expected.
So, it appears that when searching for a message (or messages) in Spark, you can only copy the link of the first message you select from that search. After that Hook gets “stuck” on that first message’s link and won’t give you any new links for any other messages in those search results.