thankyou.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <link rel="icon" href="/img//e-learn.jpg" />

    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />

    <title>Thank you</title>

    <link rel="stylesheet" href="/css/style.css" />
  </head>
  <body>
    <div class="container--1">
      <button type="submit" class="btno" onclick="openPopup()">Submit</button>
      <div class="popup" id="popup">
        <img src="/img/tick.png" alt="" />
        <h2>Thank You!</h2>
        <p>Your details has been successfully submitted. Thanks!</p>
        <a href="/contactUs.html"
          ><button type="button" class="bttn" onclick="ClosePopup()" href>
            OK
          </button></a
        >
      </div>
    </div>

    <script>
      let popup = document.getElementById("popup");
      function openPopup() {
        popup.classList.add("open-popup");
      }
      function ClosePopup() {
        popup.classList.remove("open-popup");
      }
    </script>
  </body>
</html>