He included a series of pictures showing previous secret celebrity Stormtroopers including Princes William and Harry, Daniel Craig and Tom Hardy.
Along with the hints of the hashtag #Stylestrooper and the nod to Harry's band One Direction, Mark also included a picture of Harry dressed as a Stormtrooper, from Kacey Musgraves' Halloween party.
Disney's new movies saw the return of original trilogy stars Hamill, Harrison Ford and the late Carrie Fisher as Luke Skywalker, Han Solo and Princess Leia respectively - 38 years after they made the first films.
And Mark recently revealed he was thrilled that his Empire Strikes Back and Return of the Jedi co-star Billy Dee Williams also reprised his role as Lando Calrissian for the new film.
He said: "I'm so glad Billy Dee is in the final installment, because he deserves it. I thought, 'Anybody that's still around - no matter how prominent or obscure - should come back. I would love that'.
"But especially Billy Dee. We were unknowns when we were cast, and when they cast Billy Dee in Empire, he was like a big movie star! He's just the smoothest [guy]. He's almost like an old-school, 1930s matinee idol. He's just a really wonderful guy and fun to be around. And he saved my life in Empire, so I owe him that."