The singers have signed up as ambassadors for U.K. organisation Trekstock, which offers support to young people fighting cancer.
The bandmates are asking devotees to donate to the cause for the chance to enter into a prize draw for an unforgettable trip to the British capital to hang out with them.
The duo says, "We've been working with Trekstock to come up with an amazing prize - it's something we've never, ever done before. We're offering a five-star weekend in London: We'll fly one grand-prize winner and a friend there from wherever they are in the world, and they'll spend three nights at an exclusive hotel in Central London. They'll also receive £500 in vouchers to spend on London's famous Carnaby Street before joining us both for an evening out!"
The campaign ends on 12 August (13), when one name will be chosen at random for the exclusive prize.
As an extra incentive for those who dig deep, Styles and Payne will send a thank-you email video to those who give $10 (£6.50), a special Harry/Liam bracelet to those who donate $25 (£16) or more, and a T-shirt to donors who hand over $50 (£32) or more.