Edit in JSFiddle

<!DOCTYPE html>
<html lang="it">

  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Landing Page</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
    <style>
      .sfondo {
        background: url('https://loremflickr.com/320/240?random=1') no-repeat center center;
        background-size: cover;
        height: 100vh;
      }

      .cta {
        position: relative;
        top: 35%;
        transform: translateY(-35%);
        background: rgba(0, 0, 0, 0.5);
        padding: 5rem;
      }

    </style>
  </head>

  <body>
    <div class="sfondo">
      <div class="cta container rounded text-center text-white">
        <h1>Ricevi 4 Video Lezioni Gratuite</h1>
        <p class="lead p-3">
          Sei ad un passo dalla svolta. Bastano pochi minuti.
        </p>
        <form class="form-inline justify-content-center">
          <input type="email" name="email" class="form-control form-control-lg mr-2" placeholder="La Tua Email...">
          <button type="submit" class="btn btn-lg btn-primary">Ok, Voglio Riceverli</button>
        </form>
      </div>
    </div>
  </body>

</html>