Firebase - email, password and username

iam using firebase and iam trying to add a username to the database with the email and password.

This is how my database look like now

Is there a way to do it or is createUserWithEmailAndPassword() function only for email and password?

signUp.addEventListener("click", function(user)
{
    var username = usernameTxt.value;
    var email = emailTxt.value;
    var password = passwordTxt.value;

    firebase.auth().createUserWithEmailAndPassword(email, password).catch(function(error)
    {


      // Handle Errors here.
      var errorCode = error.code;
      var errorMessage = error.message;

      if (errorCode == 'auth/email-already-in-use')
      {
            alert('email-already-in-use.');
        }
        else
        {
            alert(errorMessage);
        }
          console.log(error);

    });
});