JSFiddle - React, Tailwind, and code Playground

HTML

<form>
 <input type="button" class="addField" value="Add" />
 <table>
  <tr class="trToClone">
    <td>
      <label>Username:</label>
      <input type="text" name="username[0]" />
    </td>
    <td>
      <label>Level:</label>
      <select class="dropdown" name="level[0]">
        <option value="">---</option>
        <option value="a">A</option>
        <option value="b">B</option>
        <option value="c">C</option>
      </select>
    </td>
    <td></td>
  </tr>
 </table> 
</form>

JavaScript

var counter = 0;
$("input.addField").on('click', function() {
	counter++;
  var $tr = $('.trToClone:eq(0)');
  var $clone = $tr.clone();
  $clone.find(':text').attr('name','username['+ counter + ']');
  $clone.find('.dropdown').attr('name','level['+ counter + ']');
  $tr.parent().append($clone);
});