You can enable or disable reCaptcha for the register form from Theme Options – Advanced – reCaptcha settings.

Please note that all theme forms are already secured by default with WordPress Nonces 

To use this feature you must add site key and secret key which you take from this url https://www.google.com/recaptcha/admin/create

Choose Recaptcha V2 and add your domain name

If you get this error

Recaptcha: ERROR for site owner: Invalid domain for site key

Make sure the site url is valid, according to Google Standards (and as in the screenshot above)

A valid domain requires a host and must not include any protocol, path, port, query or fragment.

Submit the application and copy the credentials