Good Day!
I am planning to add a javascript where it will remove the onclick attribute if a certain field is empty. BTW I modify my code because I use different approach on this:
First I added a after_ui_frame logic hook and call the javascript using the custom logic hook.
        $randomNumber = rand();
        echo '<script type = "text/javascript">
        var script  = document.createElement("script");
        script.type = "text/javascript";
        script.src  = "custom/include/CustomLogicHook/clearFields.js?v=' . $randomNumber . '";
        document.body.appendChild(script);
        </script>';
And my custom JS
$("#btn_custom_city_c").attr("disabled", true);
$("#btn_custom_barangay_c").attr("disabled", true);
$('#dvt2_province_id_c').keyup(function() {
    if ($(this).val().length !=0)
        $("#btn_custom_city_c").attr("disabled", false);
    else
        $("#btn_custom_city_c").attr("disabled", true);
});
The disabled/enabled button works but it won`t work on relate field. This codes only works on a normal field
 
    