Add a reCAPTCHA element to the form.
Go to the settings for it.
At the Size setting, choose Invisible.
The Invisible reCAPTCHA requires different Site and Secret keys than the “I’m not a robot” reCAPTCHA. Create a new set of reCAPTCHA API keys at https://www.google.com/recaptcha/admin, and when creating the keys choose reCAPTCHA v2 – Invisible reCAPTCHA badge.
Once you have the keys, copy and paste them into the Forms → Settings → reCAPTCHA page.