At the moment, you can't. The week before HoW came out, Petra was in the Tower with a series of bounties, with one unlocking each day. If you completed them all then checked back in with her on Monday before the reset she gave you an invitation to the Reef, and the next day after HoW launched, you could go to her in the reef and she gave you the emblem as a reward. Unless Bungie does another event that makes it available, there's currently no way to get it.