Go to a site other then XenForo.com,
Click at the "Contact Us" link at the bottom.
Wheter you're logged in or not, just click on "Send Message".
Now wheter it's the "NO CAPTCHA" or "NO MESSAGE ENTERED" error message, there is no lightgray background, which means you have to click the X to close the message. I find it easier to click the lightgray background