The prince, 29, did so publicly on Thursday, typing out the message on an iPad at an event at London's BT Tower asking Britons to support the Invictus Games, which he helped to create – and tickets for which go on sale Friday.
Standing at the podium and tapping slowly on the tablet, Harry quipped, "That's why I joined the army – I can't type fast."
The Tweet, posted to the @InvictusLondon account, read: "Hope everyone will get behind #invictusgames. Great opportunity to support and thank the men and women who have given so much. Harry."
The Invictus Games, modeled after the U.S.'s Warrior Games, which Harry attended last year, are a Paralympic-style games for injured servicemen and -women. Some 400 competitors from 14 nations will compete in nine sports, including cycling, wheelchair rugby and sitting volleyball, at various venues over four days in September. Organizers hope to sell 40,000 tickets.
Sport plays a "hugely important part" in veterans' recovery, and the games will give "these inspirational people the opportunity to showcase their talent, and in doing so, prove that anything is really possible," Harry said at Thursday's event.
He added, "The Games do not stand in isolation but are part of a broader legacy; there are many fantastic organizations supporting those leaving the armed forces, whether injured or not. But we want to encourage many more to support where they can."
Tickets will be available at www.invictusgames.org beginning at 9 a.m. U.K. time on Friday.