Recaptcha Puzzle on contact forms

Hi,

I'm trying to add the checkbox for passing the reCAPTCHA on my website, the same checkbox that must be checked to send this message. I have generated the keys and added them to my site's configuration. It works because the badge appears in the bottom right corner, but after hours of trying, I haven't found a way to add the checkbox to the form.

Is there an easy way to add this checkbox field to the form? Do I need to include some script in my code?

Thanks,
Juan.