To help reduce spam and other forms of abuse, you can optionally enable Google reCAPTCHA to silently detect suspicious submissions and, if necessary, require the completion of a challenge to proceed.
If you haven't done so already, log in to the reCAPTCHA Admin to register the site and create your API Keys. Be sure to select "reCAPTCHA v2 → Verify requests with a challenge" when registering the site, and note that these keys can be re-used to enable reCAPTCHA on all forms on the site.
Select a Form element.
Click the Form tab.
Scroll down to Options and check Protect with reCAPTCHA.
Scroll down to reCAPTCHA.
Set Site Key and Secret Key to your previously-obtained API Keys.
(Optional) Uncheck Show badge to hide the reCAPTCHA badge that's normally displayed in the bottom-right corner of the site. Note that if you choose to do this, you must add a message before or after the form indicating its use of reCAPTCHA (learn more).
You're done! The form is now protected by Google reCAPTCHA.