How to validate TinyMCE Rich TextBox in ASP.Net

hi

I used below code instead of ckeditor

    <script src="../tinymce/tinymce.min.js"  type = "text/javascript"></script>
<script  type = "text/javascript">
    tinymce.init({
    language : 'fa',
    directionality : 'rtl',
        selector: "textarea#elm1",
    toolbar: "styleselect | bold italic | alignleft aligncenter alignright alignjustify",
       fontsize_formats: "60pt",
    style_formats: [
   
			{ title: 'بزرگ',inline: 'span',styles: { 'font': '20px tahoma'} },
            { title: 'متوسط',inline: 'span',styles: { 'font': '16px tahoma'} },
            { title: 'کوچک',inline: 'span',styles: { 'font': '14px tahoma'} },
            { title: 'کوچکتر',inline: 'span',styles: { 'font': '12px tahoma'} },
		
		],
});
</script>

and

      <div id="validError">
     <asp:RequiredFieldValidator  Display = "Dynamic" ID="RequiredFieldValidator2" runat="server" ErrorMessage="*لطفاٌ توضیحات  را وارد نمایید." ControlToValidate="elm1" CssClass="valid1A"></asp:RequiredFieldValidator>
        </div>
   <div id="Dfck2">      
            <asp:TextBox ID="elm1" name="elm1" runat="server" TextMode = "MultiLine" class="elm"></asp:TextBox>           
        </div>

here I used validation for textbox that if users don't enter text it show error...

now here problem is that when I enter text for firsttime it show validation error  I should enter text again that it doesn't show error and insert data into database.

what should I do that it doesn't show validation error when I enter text for first time it doesn't show error?

Best Regards

Neda