How to add Google Maps Autocomplete search box?

To use Google Maps/Places APIs now, you're required to use an API Key. So the API URL will change from

<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&signed_in=true&libraries=places"></script>

to

<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places"></script>

To get latitude and longitude too, you can use this simple code:

<html>
<head>
    <script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places"></script>
    <script>
        function initialize() {
          var input = document.getElementById('searchTextField');
          var autocomplete = new google.maps.places.Autocomplete(input);
            google.maps.event.addListener(autocomplete, 'place_changed', function () {
                var place = autocomplete.getPlace();
                document.getElementById('city2').value = place.name;
                document.getElementById('cityLat').value = place.geometry.location.lat();
                document.getElementById('cityLng').value = place.geometry.location.lng();
            });
        }
        google.maps.event.addDomListener(window, 'load', initialize);
    </script>
</head>
<body>
    <input id="searchTextField" type="text" size="50" placeholder="Enter a location" autocomplete="on" runat="server" />  
    <input type="hidden" id="city2" name="city2" />
    <input type="hidden" id="cityLat" name="cityLat" />
    <input type="hidden" id="cityLng" name="cityLng" />
</body>
</html>

A significant portion of this code can be eliminated.

HTML excerpt:

<head>
  ...
  <script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false&libraries=places"></script>
  ...
</head>
<body>
  ...
  <input id="searchTextField" type="text" size="50">
  ...
</body>

Javascript:

function initialize() {
  var input = document.getElementById('searchTextField');
  new google.maps.places.Autocomplete(input);
}

google.maps.event.addDomListener(window, 'load', initialize);

Just copy and paste the sameple code below.

<!DOCTYPE html>
<html>
    <head>
        <script src="https://maps.googleapis.com/maps/api/jsvv=3.exp&sensor=false&libraries=places"></script>
    </head>
    <body>
        <label for="locationTextField">Location</label>
        <input id="locationTextField" type="text" size="50">

        <script>
            function init() {
                var input = document.getElementById('locationTextField');
                var autocomplete = new google.maps.places.Autocomplete(input);
            }

            google.maps.event.addDomListener(window, 'load', init);
        </script>
    </body>
</html>

Well formatted code can be found from this link. http://jon.kim/how-to-add-google-maps-autocomplete-search-box/