Use the left-side menu on the platform to navigate to Admin > Organization Setup. Then on the top-left of the organization setup page click on Integrations scroll to External User Authentication and Provisioning > and click + Add Connector. If Okta or Ping Identity is set up, you'll see the Email-less Login option.
When Email-less Login is enabled, external user authentication will no longer require an email address when logging in. All that will be required is a unique user name or identifier defined in your external identity provider, that is no less than five characters long for the user to log into the app or web portal.
This will be evident on the web portal when navigating to the user's details page, Admin > Users & Groups > Edit User, displaying a Login ID property and the existing Email address property.
Users can enter their Login ID instead of an email address when logging in.