If you are making requests directly to the Okta API, you MUST provide the user’s actual login/username to authenticate.
If you are using a custom widget (okta-hosted or self-hosted) you can convert the username provided by the user (in this case, test123) into the actual Okta login/username before the widget makes the /authn request using the transformUsername method.
In either case though, the /authn endpoint will be sent the user’s actual username.