jQuery Simple Example
HTML
<script src="//www.addressfinder.co.nz/assets/v2/widget.js"></script>
<input type="checkbox" name="xyz[1][]" id="sel_44" style="margin:2px;" value="12345" onclick="myClick(this)">
JavaScript
$(document).ready(function() {
$('#sel_44').attr("checked", true);
});