<!DOCTYPE html>
<html>
<head>
  <title>Serch box</title>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
</head>
<body>
  <div class="middle">
    <form action="" class="search-box" method="post">
      <input type="text" class="input" name="" value="">
      <button type="button" class="btn" name="button"></button>
    </form>
  </div>

  <script type="text/javascript">
    $(".btn").on("click",function(){
      $(".input").toggleClass("inclicked");
      $(".btn").toggleClass("close");
    });
  </script>
</body>
</html>
*
{
  margin: 0;
  padding: 0;
  background: #2ecc71;
}
.middle
{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,50%);
}
.input
{
  width: 60px;
  height: 60px;
  background: none;
  border-radius: 50%;
  border: 4px solid #fff;
  box-sizing: border-box;
  outline: none;
  transition: width 0.4s ease-in-out,
        border-radius 0.8s ease-in-out,
        padding 0.2s;
  transition-delay: 0.4s;
  color: white;
  font-size: 20px;
}
.inclicked
{
  width: 360px;
  border-radius: 0;
  padding: 0 15px;
  padding-right: 40px;
}
.btn
{
  position: absolute;
  width: 60px;
  height: 60px;
  top: 0;
  right: 0;
  box-sizing: border-box;
  background: none;
  border: none;
  outline: none;
  cursor: pointer;
}
.btn::before
{
  content: "";
  width: 4px;
  height: 25px;
  background: #fff;
  position: absolute;
  transform: rotate(-45deg);
  bottom: -16px;
  right: -6px;
  transition: 0.3s;
}
.close::before,.close::after
{
  content: "";
  width: 4px;
  height: 34px;
  background: #fff;
  position: absolute;
  bottom: 12px;
  right: 28px;
  right: 28px;
}
.close::before
{
  transform: rotate(-45deg);
}
.close::after
{
  transform: rotate(45deg);
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.