https://www.gamespot.com/articles/how-to-activate-the-destiny-2-curse-of-osiris-publ/1100-6455401/

Among the additions in Destiny 2's just-released Curse of Osiris expansion is a new Public Event on Mercury. Like those in the base game, players looking for an extra challenge (or simply better rewards) can activate a more difficult, Heroic version of this activity. However, doing so isn't simply a matter of pressing a button--and it's a more involved process than of any Public Event that we've seen before. Here's what you need to do.