This way
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>TinyMCE ASP.Net Example</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<script type="text/javascript" src="//tinymce.cachefly.net/4.0/tinymce.min.js"></script>
<asp:TextBox ID="txtTinyMCE" TextMode="MultiLine" runat="server" />
<asp:RequiredFieldValidator runat="server" ControlToValidate="txtTinyMCE" ErrorMessage="Required"
ForeColor="Red" />
<asp:CustomValidator ErrorMessage="Max 10 characters allowed." ForeColor="Red"
ClientValidationFunction="ValidateCharacterLength" runat="server" />
<asp:Button ID="btnSave" runat="server" Text="Save" />
</div>
<script type="text/javascript">
window.onload = function () {
tinymce.init({ selector: 'textarea' });
}
function ValidateCharacterLength(sender, args) {
var body = tinymce.get("<%=txtTinyMCE.ClientID %>").getBody();
var content = tinymce.trim(body.innerText || body.textContent);
args.IsValid = content.length <= 100;
}
</script>
</form>
</body>
</html>
