Presumably after the fact by linking them entirely within Jira. You could mention the commit in the ticket, but that wouldn't make a back-link from the commit if someone's browsing from that direction.

That said, I don't really mind mentioning the ticket in the commit. It's relevant to what one is doing when one makes the commit.

But then I use gerrit at my job, so I don't have room to complain about others' systems.