Please refer below code.
HTML
<div>
<title>TinyMCE ASP.Net Example</title>
<script type="text/javascript" src="tinymce/jscripts/tiny_mce/tiny_mce.js"></script>
<script type="text/javascript">
tinyMCE.init({
mode: "textareas",
theme: "advanced",
plugins: "safari,spellchecker,pagebreak,style,layer,table,save,advhr,advimage,advlink,emotions,iespell,inlinepopups,insertdatetime,preview,media,searchreplace,print,contextmenu,paste,directionality,fullscreen,noneditable,visualchars,nonbreaking,xhtmlxtras,template,imagemanager,filemanager",
theme_advanced_buttons1: "save,newdocument,|,bold,italic,underline,strikethrough,|,justifyleft,justifycenter,justifyright,justifyfull,|,styleselect,formatselect,fontselect,fontsizeselect",
theme_advanced_buttons2: "cut,copy,paste,pastetext,pasteword,|,search,replace,|,bullist,numlist,|,outdent,indent,blockquote,|,undo,redo,|,link,unlink,anchor,image,cleanup,help,code,|,insertdate,inserttime,preview,|,forecolor,backcolor",
theme_advanced_buttons3: "tablecontrols,|,hr,removeformat,visualaid,|,sub,sup,|,charmap,emotions,iespell,media,advhr,|,print,|,ltr,rtl,|,fullscreen",
theme_advanced_buttons4: "insertlayer,moveforward,movebackward,absolute,|,styleprops,spellchecker,|,cite,abbr,acronym,del,ins,attribs,|,visualchars,nonbreaking,template,blockquote,pagebreak,|,insertfile,insertimage",
theme_advanced_toolbar_location: "top",
theme_advanced_toolbar_align: "left",
theme_advanced_statusbar_location: "bottom",
theme_advanced_resizing: false,
template_external_list_url: "js/template_list.js",
external_link_list_url: "js/link_list.js",
external_image_list_url: "js/image_list.js",
media_external_list_url: "js/media_list.js",
max_height: 500,
max_width: 500,
min_height: 100,
min_width: 400,
height: 500,
width: 500
});
</script>
<asp:TextBox ID="RichTextBox" runat="server" TextMode="MultiLine" Style="width: 100px;
height: 100px"></asp:TextBox>
<asp:Button ID="btnSave" runat="server" Text="Save" OnClick="btnSave_Click" />
<asp:Button ID="btnCancel" runat="server" Text="Cancel" OnClick="btnCancel_Click" />
<asp:Label ID="lblDisplay" runat="server" Text="" Visible="false"></asp:Label>
</div>
I hope this will help you out.