Go to www.google.com/recaptcha/about/, setup a new domain, and save the values given e.g. to Heroku for Heroku deployment:
heroku config:set -a ourbigbook RECAPTCHA_SECRET_KEY=secret_key
heroku config:set -a ourbigbook NEXT_PUBLIC_RECAPTCHA_SITE_KEY=site_key
Aditionally, also setup a separate localhost reCAPTCHA to test that it is working:
echo RECAPTCHA_SECRET_KEY=secret_localhost_key >> web/.env
echo NEXT_PUBLIC_RECAPTCHA_SITE_KEY=site_localhost_key >> web/.env
Although it is possible to use a single reCAPTCHA for both production and development, Google recommends having separate ones.
If the NEXT_PUBLIC_RECAPTCHA_SITE_KEY variable is not set, then reCAPTCHA is simply not used in the website.