LONDON—A boom in corporate dealmaking, surging input costs, and a focus on short-term cash flows in the pandemic have sent companies rushing to hedge their currency exposures this year, giving a boost to banks that sell foreign exchange products. Corporate treasurers say the pandemic, which sent revenues tanking in 2020 before this year’s sharp rebound, has encouraged many to hedge currency risks more frequently. Relentless supply chain pressures, and a sharp rise in raw material and other input costs that are mostly denominated in U.S. dollars, are reasons for companies to lock in prices too. And a surge in mergers and acquisitions as the recovery takes hold is also lifting corporate demand for foreign currencies. Global dealmaking is running at a record high this year, with $3.9 trillion of deals already transacted by early September, according to Refinitiv data. Multinational firms are among those to have increased their foreign exchange …