Place inside <head>:
<script src="https://primopostcodes.com/cdn/primopostcodes-typeahead-2.0.min.js">
</script>
Place after </form>:
<script>
primoPostcodes.typeahead('typeahead-postcode','postcode','[ YOUR API KEY ]');
primoPostcodes.typeahead('typeahead-town-or-city','town_or_city','[ YOUR API KEY ]');
primoPostcodes.typeahead('typeahead-county','county','[ YOUR API KEY ]');
primoPostcodes.typeahead('typeahead-country','country','[ YOUR API KEY ]');
</script>
Choose a plan to receive your API Key.