Hi irshad1231,

Check this example. Now please take its reference and correct your code.

HTML

<script src="https://cdn.ckeditor.com/4.16.1/standard-all/ckeditor.js"></script>
<asp:TextBox ID="editor1" runat="server" TextMode="MultiLine"></asp:TextBox>
<br />
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
<script>
    CKEDITOR.replace('editor1', {
        extraPlugins: 'embed,autoembed,image2',
        height: 500,
        // Load the default contents.css file plus customizations for this sample.
        contentsCss: [
            'https://cdn.ckeditor.com/4.16.1/full-all/contents.css',
            'https://ckeditor.com/docs/ckeditor4/4.16.1/examples/assets/css/widgetstyles.css'
        ],
        // Setup content provider. See https://ckeditor.com/docs/ckeditor4/latest/features/media_embed
        embed_provider: '//ckeditor.iframe.ly/api/oembed?url={url}&callback={callback}',
        // Configure the Enhanced Image plugin to use classes instead of styles and to disable the
        // resizer (because image size is controlled by widget styles or the image takes maximum
        // 100% of the editor width).
        image2_alignClasses: ['image-align-left', 'image-align-center', 'image-align-right'],
        image2_disableResizer: true
    });
</script>

Code

C#

protected void Button1_Click(object sender, EventArgs e)
{
    string CkEditorText = editor1.Text;
    // Insert code.
}

VB.Net

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim CkEditorText As String = editor1.Text
End Sub

Note: Set ValidateRequest="false" inpage level and add requestValidationMode in web config.

<httpRuntime requestValidationMode="2.0" />

Refer below article.

ASP.Net Error: A potentially dangerous Request.Form value was detected from the client