First, ensure your site is accessible from an external domain, as Google will need this.
Visit Register a new site. Fill in the label, add your domain(s) (just the domain, no subpath, protocol or port), and Choose the “Challenge (v2) / I’m not a robot Checkbox” reCAPTCHA type.
Click submit. On the next page, copy both the Site Key and Secret Key and keep them for later.
Go to jfa-go Settings > Captcha, and enable reCAPTCHA. Paste in the site key and secret key from above, and fill in the hostname to match the one you gave Google.
Save & restart jfa-go, and you should be good to go.