If I am making a username password login screen, when the user writes his username and clicks on the password (to be exact when the user clicks out of the username input), I want to check if the username exists in my external database, If it does, I want nothing to happen, But if it doesn't I want the label to act the same as when you have a required field and leave it empty (Warning, Value cannot be blank).

the first part is achievable via action, however is the second part (adding 'Value cannot be blank' and making the label red on the user input) possible?

Thanks.