Another thing I always have to lookup while it is so simple....
1. install the captcha and image captcha module
2. go to configuration > people > captcha
3. Add form id
Add the form id of the forms to which you want to add a captcha challenge on the bottom of this page and select the captcha type
You can find the id by opening the form an looking in the HTML. There you find an id like: webform-client-form-75
<form accept-charset="UTF-8" id="webform-client-form-75" method="post" action="/en/order" enctype="multipart/form-data" class="webform-client-form">
NB! Replace all hyphens by underscores!
webform-client-form-75 -> webform_client_form_75
If you have a multilingual site and want to translate the description, you have to do this partly on the capcha configuration page, partly via the normal Drupa interface translation.
(which is rather stupid)
For instance, as I am Dutch (although also half Frysian) I always translate
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.
To a bit archaic
Deze vraag wordt gebruikt om te testen indien u een menselijke bezoeker bent teneinde spam-inzendingen te vermijden.
What code is in the image? *To
Welke code ziet u in de afbeelding?: *
Enter the characters shown in the image.
Which is more or less a useless repetition of the previous sentence I think, to
Vul bovenstaande code in.
5. State of Bliss
And then, your webform will contain a captcha. Rejoice!
For Drupal 8: see this post