If you're logged in, there's a "Vote for this issue" link next to the number of votes in the info box at the top of the issue. (The UI will strongly discourage "+1" comments unless they have something else to say.)

Welp, good luck to them in beating GitHub, haha.

Edit: the answer is to use JIRA, since they are reasonably well integrated in the on-prem solutions. For their site, no idea.