Ecco a te:

codice:
<script>
function padStart(value, targetLength,padString) {
  targetLength = targetLength>>0; //floor if number or convert non-number to 0;
  padString    = String(padString || ' ');


  if (value.length > targetLength) {
    return String(value);
  }
  else {
    targetLength = targetLength-value.length;


    if (targetLength > padString.length) {
      padString += padString.repeat(targetLength/padString.length); //append to original to ensure we are longer than needed
    }


    return padString.slice(0,targetLength) + String(value);
  }
}
//funzione​
function check_numero()
{
var numero = $("#numero").val();


  $("#numero").val(String("00000000" + numero).slice(-8));
  numero =$("#numero").val();
};
</script>
HTML

<div class="form-group" id="box_numero">
                    <label >Numero</label> <span style="color:#ff0000">*</span> <div id="check_mc"class="pull-right"></div>
                     <input type="text" class="form-control" name="num" id="numero" autocomplete="off" onkeyup="check_numero();" required ><!-- data-inputmask='"mask": "99999999"' data-mask-->
        </div>