I have just tested this on my side on a Sitefinity 7.1 project following the exact steps from the KB article and the captcha is validated properly.
Please download the archive from the KB article and extract it in the root folder of your project solution. The archive contains the SubscribeFormCustom.cs
file which is extending the custom Subscribe form widget. The SubscribeFormCustomTemplate.ascx
file in the archive is the custom template that the custom subscribe form widget will use where I have added the captcha control in the markup.
Then please build the project and register the custom subscribe form (SubscribeFormCustom.cs)
in the Toolboxes using Thunder as described in this article: http://docs.sitefinity.com/thunder-register-a-widget
Restart the application in order for the changes to take effect and you will be able to drop the custom Subscribe form on a page and test the behavior with the captcha.
I am also sending attached a video for your reference demonstrating the results on my side. As you can see from the video the overridden AddSubscriber() method is called and if the captcha is not valid the base AddSubscriber() method is not executed and the subscriber is not added to the mailing list.
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