Confusing UI login advice on a cloud instance

Our users are often confused by the login UI hint as presented on the login page.

The UI hint instruction says new users ("registrants" ?) to use e-mail address, however:

• Nowhere in our cloud instance's login page is there a link for end users to self-register, users need to be added by an admin (which is our standard process)
• Once a new user is added by instance administrator, they need to authenticate using username and password
• When they follow the advice from UI ("For new registrants, use the email you provided during registration"), they tend to enter e-mail address which doesn't work and  results in authentication failure.
  • That's because the term "registrant" is not clear to end users (and to me as instance administrator either to be fair); since advice also mentions "new", they assume that their first login needs to made with an e-mail address.

Could this be made clearer in the UI?