Refer the below code.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
$(function () {
$('[id*=txtName]').on('focus', function () {
$('[id*=btnSave]').show();
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<input name="txtName" type="text" id="txtName" />
<br />
<br />
<input type="submit" name="btnSave" value="Save" id="btnSave" style="display: none;" />
</div>
</form>
</body>
</html>
Demo
You can replace html control with server controls.
<div>
<asp:TextBox runat="server" ID="txtName" />
<br />
<asp:Button Text="Save" runat="server" ID="btnSave" Style="display: none;" />
</div>
