Hi Adnan,
Do this way. You can use the JavaScript window.onload event handler which is called when the Page loading is complete in Browser.
Pls check the below sample.
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
body { font-family: Arial; font-size: 10pt; }
</style>
</head>
<body>
<script type="text/javascript">
window.onload = function(){
alert("Page has been loaded!");
};
</script>
</body>
</html>
Demo