Think that this spring’s “Robin Hood” movie will be a blockbuster at the box office? Next week you will be able to put your money on it. Cantor Futures Exchange, a subsidiary of Cantor Fitzgerald, expects to open an online futures market next month that will allow studios, institutions and moviegoers to place bets on the box-office revenue of Hollywood’s biggest releases. Last week, the company learned from regulators that customers could start putting money into their accounts on March 15.