You don’t normally need a username or password to get into eM Client.
However if you are locked out of eM Client and cannot get into the program / gui “due to you have setup a username and password on eM Client startup”, then see the following thread.
Now if you can get into eM Client ok, but are getting asked for an account username and password, then you will need to contact your Mail Server technical support via phone or website to get your password reset.