JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/jquery-ui.min.js"></script>

  <div id="st-box">
    <input type="text" name="ref_name[]" class="st-reg-step2 no-line" placeholder="Please."/>
    <input type="phone" name="ref_phone[]" class="st-reg-step2 no-line" placeholder="Please ."/>
   </div>
  <span id="add-btn">Add+</span>

JavaScript

$('#add-btn').click(function(){
	$(this).prev("#st-box").clone().insertBefore(this);
})