Oppure, se vuoi toglierti di mezzo jQuery...

codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Documento HTML</title>
<script type="text/javascript">
var btnClick = (function () {

	function AJAXSuccess () {
		document.getElementById("risultato").value = this.responseText;
		oGhost.parentNode && document.body.removeChild(oGhost);
	}

	function AJAXError (oError) {
		alert("Chiamata fallita, si prega di riprovare...");
		oGhost.parentNode && document.body.removeChild(oGhost);
	}

	var oGhost = document.createElement("div"), oContainer = document.createElement("div"), oWhait = new Image();

	oGhost.id = "ghost";

	/* inserisci qui l'url dell'immagine di attesa.... */
	oWhait.src = "https://www.google.it/images/srpr/logo4w.png";

	oContainer.appendChild(oWhait);
	oGhost.appendChild(oContainer);

	return function () {
		var oReq = new XMLHttpRequest();
		oReq.onload = AJAXSuccess;
		oReq.onerror = AJAXError;
		oReq.open("post", "mailurltrovata.php", true);
		oReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
		oReq.send("url=" + escape(document.getElementById("url").value) + "&numpag=" + escape(document.getElementById("numpag").value));
		oGhost.parentNode || document.body.appendChild(oGhost);
	};

})();

</script>
<style type="text/css">
	#ghost {
		position: fixed;
		left: 0;
		top: 0;
		display: table;
		background-color: #000000;
		opacity: 0.5;
		vertical-align: middle;
		width: 100%;
		height: 100%;
	}

	#ghost div {
		display: table-cell;
		vertical-align: middle;
		text-align: center;
		width: 100%;
		height: 100%;
	}

	#ghost div img {
		display: block;
		margin: auto;
	}
</style>
</head>
 
<body>

<form name="tuoForm">

	

URL: <input type="text" name="indirizzo" id="url" />

	Numero di pagina: <input type="text" name="pagina" id="numpag" /></p>

	

<textarea id="risultato"></textarea></p>

	

<input type="button" id="bottone" value="Cliccami" onclick="btnClick();" /></p>

</form>

</body>
</html>