When we click the "Create Account" link (which is only available for tenants for multi-tenant applications) in the login page, a registration form is shown:
Recaptcha (security question) is optional. It uses Google's Recaptcha service. Recaptcha service works per domain. So, to make it properly work, you should create your own private and public keys for your domain on https://www.google.com/recaptcha and replace keys in appsettings.json file under the *.Web.Host project and in the appconfig.json in the client side.
Each tenant can enable/disable user registration under the "User management" tab of settings page. If user registration is disabled for a tenant, "Create Account" link will not be visible on the login page for that tenant.