This plugin supports the reCAPTCHA v2 "I'm not a robot" Checkbox which will exist on your HTML form:
Start by creating a site or using an existing site in the reCAPTCHA Admin Console. Retrieve the site and secret keys from the site's settings page:
Follow the reCAPTCHA docs to add the reCAPTCHA v2 widget to your site's HTML form.
When the user submits the form, Postform will verify the user's checkbox response token by checking the
g-recaptcha-response POST parameter.
In order for Postform to verify the user's response, it needs to know your site's secret key. Copy the secret key from the site's settings page (shown above) and paste it in Postform's configuration dialog: