code: function onSignInWithCustomToken(event) {
    // The token can be directly specified on the html element.
    var token = $('#user-custom-token').val();
  
    auth.signInWithCustomToken(token)
        .then(onAuthUserCredentialSuccess, onAuthError);
  }
docs: Signs in with a custom token.
  @param {DOMEvent} event HTML DOM event returned by the listener.