It happened at the at the "Dunkirk" premiere at the Odeon in Leicester Square. It's the 1D guy's debut role in a movie, and it happens to be about the WWII battle at Dunkirk -- which explains why the royal one was all ribboned up.
It's not the first time they've met -- as many assumed. They hooked up in 2015 at a Royal concert event in London. Y'know the red headed Harry's jammed out to "Story of My Life" more than a few times, right?