例えば、
src/Eccube/Resource/template/default/Entry/index.twig
なら
{% block javascript %}
<script src="//ajaxzip3.github.io/ajaxzip3.js" charset="UTF-8"></script>
<script>
$(function() {
$('#zip-search').click(function() {
AjaxZip3.zip2addr('entry[zip][zip01]', 'entry[zip][zip02]', 'entry[address][pref]', 'entry[address][addr01]');
});
});
</script>
{% endblock javascript %}
(snip)
<div id="top_box__zip" class="form-group form-inline input_zip {% if form.zip.zip01.vars.errors is not empty or form.zip.zip02.vars.errors is not empty %}has-error{% endif %}">{{ form_widget(form.zip) }}</div>
が、
html
<div id="top_box__zip" class="form-group form-inline input_zip ">〒<input type="text" id="zip01" name="entry[zip][zip01]" required="required" style="ime-mode: disabled;" pattern="\d*" class="form-control" />-<input type="text" id="zip02" name="entry[zip][zip02]" required="required" style="ime-mode: disabled;" pattern="\d*" class="form-control" /> <span class="question-circle"><svg class="cb cb-question"><use xlink:href="#cb-question" /></svg></span> <a href="http://www.post.japanpost.jp/zipcode/" target="_blank">郵便番号検索</a><div class="zip-search"><button type="button" id="zip-search" class="btn btn-default btn-sm">郵便番号から自動入力</button></div></div>
(snip)
<script src="//ajaxzip3.github.io/ajaxzip3.js" charset="UTF-8"></script>
<script>
$(function() {
$('#zip-search').click(function() {
AjaxZip3.zip2addr('entry[zip][zip01]', 'entry[zip][zip02]', 'entry[address][pref]', 'entry[address][addr01]');
});
});
</script>
と、変換されて、「AjaxZip3.zip2addr()」が"//ajaxzip3.github.io/ajaxzip3.js"からリアルタイムに住所を取得します。