limit username domain extension to specific domains
I need to restrict the Okta Login username to specific domain extensions. When creating a user account, only the domains I require should be allowed in the Username field - e.g. "user1@domainA.com" or "user1@domainB.com". If any other domain extension is specified, creation of the account is prevented.
You can't do that as that feature does not exist currently. Your question implies that your users are not Okta Mastered since you are mentioning domainA.com or domainB.com. If it is AD mastered, user accounts are typically created in AD and imported to Okta and as such the user account will assume the right domain extension based on the domain the user account was created in AD. If you are Okta mastered, that feature is currently unavailable.