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 your site and create your API Keys. Be sure to select "reCAPTCHA v2 → Invisible reCAPTCHA badge" when registering your site, and note that these keys can be re-used to enable reCAPTCHA on all forms on your site.
Select your Form element.
Click the Form tab.
Scroll down to reCAPTCHA and check Enable reCAPTCHA protection.
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 your site. Note that if you choose to do this, you must add a message before or after your form indicating its use of reCAPTCHA (learn more).
You're done! Your form is now protected by Google reCAPTCHA.