title: How to make DotNetNuke Captcha control to show an error if it left blank
body: <p>I took a captcha control on my page :</p>
  
  <pre><code> &lt;dnn:CaptchaControl ID="CaptchaControl1" runat="server" CaptchaHeight="50px"    CaptchaWidth="150px" ErrorMessage="incorrect"     /&gt;
  </code></pre>
  
  <p>if captcha is incorrect the error message will be shown but if it left blank it shows no error.is there any way to make it show an error when it is blank?</p>
  
  <p>thanks</p>