There is MouseEvent.button, which is 1 which is for “Auxiliary button pressed, usually the wheel button or the middle button (if present)”.
I think this is fine. I don’t think we want to count strictly the amount of times the linked page was loaded, rather just “link interactions”. I think “copy link” should also increase the counter, just as simply left clicking the link does.
In addition, I think most people don’t simply left-click such links: they usually want to open it in a new tab such that the original forum post is still open and they don’t have to navigate back and forth.