HTML - Ayuda Numero Aleatorio en Form o Body

<div>

		<form action=""enviar.php" method="post">

			<script type="text/javascript">

			var numerorandom;

			numerorandom = Math.random()*10000;

			document.getElementById("numeroaleatorio").value = variable;

			</script>

			<input type="hidden" name="numerorandom" id="numeroaleatorio">

			<input type="text" name="name" id="name" class="name" placeholder="NOMBRE"><br>

			<input type="text" name="mail" id="mail" class="email" placeholder="CORREO"><br>

			<input type="text" name="phone" id="subject" class="subject" placeholder="TELÉFONO"><br>

			<textarea name="message" class="message" placeholder="MENSAJE ... " id="message"></textarea><br>

			<div class="clear"></div>

			<div class="col-md-8">

			<button type="submit" class="btn btn-primary">ENVIAR </button>

			</div>

		</form>

</div>