Hi Adnaan,
Inside CheckBox click event handler, check if CheckBox is checked then show else hide the TextBox by setting the style attribute to block and none respectively.
Refer below example.
HTML
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
body { font-family: Arial; font-size: 10pt; }
</style>
</head>
<body>
<span>Do you have Passport?</span>
<input id="chkConfirm" type="checkbox" onclick="ShowHideTextBox()" /><hr />
<input id="txtPassportNumber" type="text" style="display:none" />
<script type="text/javascript">
function ShowHideTextBox() {
document.getElementById("txtPassportNumber").style.display =
document.getElementById("chkConfirm").checked ? "block" : "none";
}
</script>
</body>
</html>
Demo
