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