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Firstly go to https://www.google.com/recaptcha/admin. Sign up if needed and then go to the "Register a site section".
Label: Anything you wish.
Choose the type of reCAPTCHA: Recaptcha V2
Domain: Your blesta domain eg: yourdomain.com or billing.yourdomain.com
Accept the terms and save.
You can then go to the table above and click the row for your label.
You will see this: Adding reCAPTCHA to your site
Click on keys and then add the two keys to
Packages > Order forms > Settings > Human verification