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 

If enabled – the register form looks like this:

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

Click on Get Recaptcha

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

Get the site key and secret key

Copy/paste in theme options