getAddress()

<!-- Postcode field -->
<div id="postcode_lookup"></div>  

<!-- Add to your existing form -->
<label>First Address Line</label>
<input id="line1" type="text"> 

<label>Second Address Line</label>
<input id="line2" type="text">   

<label>Third Address Line</label>
<input id="line3" type="text">  

<label>Town</label>
<input id="town" type="text">

<label>County</label>
<input id="county" type="text">

<label>Postcode</label>
<input id="postcode" type="text">
                            

<!-- Include JQuery v2+ -->
<script src="jquery.js"></script>
<!-- Include plugin file -->
<script src="https://getaddress-cdn.azureedge.net/scripts/jquery.getAddress-3.0.4.min.js"></script>

<!-- Add after your form -->
<script>
$('#postcode_lookup').getAddress(
    {
    api_key: 'YOUR_API_KEY',  
    output_fields:{
        line_1: '#line1',
        line_2: '#line2',
        line_3: '#line3',
        post_town: '#town',
        county: '#county',
        postcode: '#postcode'
    }
});
</script>