Monzo, an online retail bank in the U.K. popular with millennials, will announce a buy-now-pay-later product on Thursday, jumping into the crowded market dominated by the likes of Klarna, Affirm and Afterpay. The British bank’s product dubbed “Monzo Flex” will give users the opportunity to split purchases across several payments over a three, six or 12 month time period, according to a person...