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 plugin file -->
<script src="https://getaddress.io/js/jquery.getAddress-2.0.5.min.js"></script>

<!-- Add after your form -->
<script>
$('#postcode_lookup').getAddress({
    api_key: 'YOUR_API_KEY',  
    <!--  Or use your own endpoint - api_endpoint:https://your-web-site.com/getAddress, -->
    output_fields:{
        line_1: '#line1',
        line_2: '#line2',
        line_3: '#line3',
        post_town: '#town',
        county: '#county',
        postcode: '#postcode'
    },
<!--  Optionally register callbacks at specific stages -->                                                                                                               
    onLookupSuccess: function(data){/* Your custom code */},
    onLookupError: function(){/* Your custom code */},
    onAddressSelected: function(elem,index){/* Your custom code */}
});
</script>