<header>
  <div class="container">
    <div id="search-field-wrapper">
      <form action="" method="get" id="globalSearch" role="search" class="hidden-xs">
        <span id="scope-selector">&nbsp;</span>
        <input type="text" class="search-field" placeholder="Search our website..." id="tnaSearch" name="search_text" required aria-required="true">
        <input type="submit" class="search-button" id="search-button" value="">
      </form>
      <ul>
        <li><a title="Search our website" href="#" class="formDestinationChanger" data-target="" data-placeholder="Search our website..." data-fieldName="search_text" role="button" aria-label="Change form destination to search the website">Search our website</a></li>
        <li><a title="Search our catalogue for records" href="#" class="formDestinationChanger" data-target="
 " data-placeholder="Search our records..." data-fieldName="_q" role="button" aria-label="Change form destination to search the catalogue">Search our records</a></li>
      </ul>
    </div>
    
  </div>
</header>
body {
  background:#222
}
header {
  position:relative;
  max-width:300px;
  margin:0 auto;
}
.container {
  margin-top:40%;
}
#search-field-wrapper input[type=text] {
  position: absolute;
  top: 0;
  right: 55px;
  width: 226px;
  height: 40px;
  color: #222
}

#search-field-wrapper input[type=submit] {
  position: absolute;
  right: 15px;
  width: 40px;
  height: 40px;
  background-image: url(https://nationalarchives.gov.uk/wp-content/themes/tna-base/img/search-white.png);
  background-repeat: no-repeat;
  background-position: center
}

#search-field-wrapper ul {
  background-color: #333;
  position: absolute;
  top: 41px;
  right: 118px;
  padding: 5px;
  width: 202px;
  z-index: 4000
}

#search-field-wrapper ul li {
  list-style-type: none;
  list-style-image: none
}

#search-field-wrapper ul li a {
  color: #fff;
  font-size: .9em;
  padding: .5em;
  display: block
}

#search-field-wrapper ul li a:hover {
  background-color: #FCE45C;
  color: #222
}

header #scope-selector {
  display: inline-block;
  position: absolute;
  top: 0;
  right: 281px;
  width: 40px;
  height: 40px;
  line-height: 40px;
  background-image: url("https://nationalarchives.gov.uk/wp-content/themes/tna-base/img/scope-selector.png");
  background-color: #555;
  background-size: cover;
  cursor: pointer;
}

#globalSearch .expanded {
  background-image: url("https://nationalarchives.gov.uk/wp-content/themes/tna-base/img/scope-selector-expanded.png");
}

.search-field-wrapper-medium {
  float: right;
  max-width: 331px;
  margin-right: 1em;
}

.search-field-wrapper-medium input[type="text"] {
  min-width: 70%;
}

#changeDestination {
  color: #fff;
  padding-top: 0.5em;
}

#changeDestination label {
  margin: 0 0.5em;
}

#search-field-wrapper input[type=submit] {
  position: absolute;
  right: 15px;
  width: 40px;
  height: 40px;
  background-image: url("https://nationalarchives.gov.uk/wp-content/themes/tna-base/img/search-white.png");
  background-repeat: no-repeat;
  background-position: center;
}

input[type=submit] {
  background-color: #008484;
  color: #fff;
  padding: .3em .5em;
  border: 0;
  height: 40px;
  margin: 0;
}

input.width-half,
input[type=text] {
  border: 1px solid #ccc;
  padding: 2px;
  height: 34px;
  text-indent: 5px;
  min-width: 220px;
  width: auto;
}

#mega-menu-mobile {
    height: 40px;
    width: 40px;
    background: url(../img/menu-trigram-lrg.png) center no-repeat #cb0d07;
    border: 0;
    z-index: 2;
    border-radius: 100%;
    float: left;
    display: block;
    cursor: pointer;
    overflow: hidden;
    position: relative;
}
    // ----------------------------------------
    // Search our website -------------------

    // Hide search option
    $('#search-field-wrapper ul').hide();
    // $.showExpander()
    $.showExpander = function() {
      if ($('.expander').length) {
        $('.expander').delay(200).slideDown(400);
      }
    };

    // $.customEventer()
    $.customEventer = function(passedObject) {
      var elementIdOrClass = passedObject.elementIdOrClass,
        eventToWatch = passedObject.eventToWatch,
        customEventToTrigger = passedObject.customEventToTrigger;

      $(document).on(eventToWatch, elementIdOrClass, function() {
        $(document).trigger(customEventToTrigger);
      });
    };

    // Generic toggle method. Does not include any bindings since it is designed to be used with
    $.toggleDisplayOfElement = function(toggler, togglee) {
      $(togglee).toggle();
      $(toggler).toggleClass('expanded');
    };

    $('.formDestinationChanger').on('click', function() {
      var placeholder = $(this).attr('data-placeholder'),
        target = $(this).attr('data-target'),
        fieldName = $(this).attr('data-fieldName');

      $.toggleDisplayOfElement('#scope-selector', '#search-field-wrapper ul');

      $('#tnaSearch').attr({
        'placeholder': placeholder,
        'name': fieldName
      });
      $('#globalSearch').attr('action', target);
    });

    // When click change the arrow position
    $('#scope-selector').click(function() {
      $('#search-field-wrapper ul').toggle();
      if ($('#search-field-wrapper ul:visible').size() != 0) {
        $(this).addClass('expanded');
      } else {
        $(this).removeClass('expanded');
      }
    });

    $(document).on('click', '#search-controls li', function(e) {
      $('#search-controls li').removeClass('selected');
      $(e.target).addClass('selected');
    });

    // Global search - larger screens
    $(document).one('toggleSearchOptionsOnce', function() {
      $.toggleDisplayOfElement('#scope-selector', '#search-field-wrapper ul');
    });

    $(document).on('toggleSearchOptions', function() {
      $.toggleDisplayOfElement('#scope-selector', '#search-field-wrapper ul');
      $(document).off('toggleSearchOptionsOnce');
    });

    // Global search - smaller screens
    $(document).on('change', '.mobileSearchDestinationOption', function() {
      var target = $(this).attr('data-target'),
        placeholder = $(this).attr('data-placeholder'),
        fieldName = $(this).attr('data-fieldName');
      $('#mobile-search-field').attr({
        'placeholder': placeholder,
        'name': fieldName
      }).focus();
      $('#mobileGlobalSearch').attr('action', target);
    });

    // ----------------------------------------
    // ----------------------------------------

External CSS

  1. https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css

External JavaScript

  1. //cdnjs.cloudflare.com/ajax/libs/jquery/2.2.2/jquery.min.js