Am using this Example link https://www.aspsnippets.com/Articles/Show-number-of-characters-remaining-in-TextArea-in-real-time-using-JavaScript-and-jQuery.aspx
 
in my side Counting letter is not working
<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 () {
           
 
            //Specifying the Character Count control explicitly
            $("[id*=TextBox1]").MaxLength(
            {
                MaxLength: 15,
                CharacterCountControl: $('#counter')
            });
 
           
        });
    script>
head>
<body>
    <form id="form1" runat="server">
    <div id="counter">
    div>
    <asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine" Width="300" Height="100"
        Text="Mudassar Khan">asp:TextBox>
    
    
    form>
body>
html>