Hi pradip.kalsar,
Please refer below sample.
HTML
<asp:TextBox ID="txtDetails" runat="server" TextMode="MultiLine"></asp:TextBox>
<script type="text/javascript" src="https://cdn.ckeditor.com/ckeditor5/35.0.1/classic/ckeditor.js"></script>
<script type="text/javascript">
ClassicEditor.create(document.querySelector('#txtDetails'));
</script>
Screenshot