Генерация чисел в html

document.getElementsByClassName('input-text')[0].value= "bill[" + parseInt(Math.random() * 1000000000000000) + "]";

<div class="checkout-billing-content-field checkout-billing-content-total"> 
  <div class="checkout-billing-content-title"> 
    <span>Примечание</span> 
  </div> 
  <div class="checkout-billing-content-amount"> 
    <span class="checkout-currency"> 
      <input type="text" class="input-text" style="margin-right: 50px;" readonly > - вместо этих символов нужна генерация случайных чисел 
    </span> 
  </div> 
</div>

$(function(){ 
  var text = ""; 
  var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"; 
 
  for(var i=0; i < 10; i++) 
  { 
    text += possible.charAt(Math.floor(Math.random() * possible.length)); 
  } 
   
  $(".input-text").val("bill[" + text + "]"); 
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<div class="checkout-billing-content-field checkout-billing-content-total"> 
  <div class="checkout-billing-content-title"> 
    <span>Примечание</span> 
  </div> 
  <div class="checkout-billing-content-amount"> 
    <span class="checkout-currency"> 
      <input type="text" class="input-text" onclick="$(this).select();" style="margin-right: 50px;" readonly value="bil[mnkOmfrNCJROaRw]"> 
    </span> 
  </div> 
</div>

(function() { 
  var num = Math.round((Math.random() * 100) * (Math.random() * 100)); 
  document.querySelector('.input-text').value = 'bill['+num+']'; 
})();

<div class="checkout-billing-content-field checkout-billing-content-total"> 
  <div class="checkout-billing-content-title"> 
    <span>Примечание</span> 
  </div> 
  <div class="checkout-billing-content-amount"> 
    <span class="checkout-currency"> 
      <input type="text" class="input-text" onclick="$(this).select();" style="margin-right: 50px;" readonly value="bill[]"> 
    </span> 
  </div> 
</div>

var min = +prompt("Min: ", ""); 
var max = +prompt("Max: ", ""); 
function randomInteger(min, max) 
{ 
    var result = min + Math.random() * (max + 1 - min); 
    result = Math.floor(result); 
    return result; 
} 
alert(randomInteger(min, max));