Hola lexus.
Prueba con esto:
Código PHP:
<html>
<head>
<script language="JavaScript">
function urlComplete(){
with (document.frm.urlVal)
value = (value.substr(0, 7) == "http://"? "": "http://") + value;
}
</script>
</head>
<body>
<form name="frm">
URL:
<input type="text" name="urlVal"><br><br>
<input type="button" value="Validar" onclick="urlComplete()">
</form>
</body>
</html>
Saludos.