I was busy nijas, I suggest the following:

Make debug as true to disable lazyload.

<compilation defaultLanguage="c#" debug="true"

In the path folder yourapp/Umbraco/Js/umbraco.directives.js insert the alert as in the code below:

vm.loginStates.submitButton = 'busy';
userService.authenticate(login, password).then(function (data) {
 // Demo for nijas
alert('User logged in before redirect to backoffice');
vm.loginStates.submitButton = 'success';
userService._retryRequestQueue(true);
if (vm.onLogin) {
vm.onLogin();
}

In the path folder yourapp/Umbraco/Js/umbraco.controllers.js insert the alert as in the code below:

dashboardResource.getDashboard($routeParams.section).then(function (tabs) {
$scope.dashboard.tabs = tabs;
            // set first tab to active
            if ($scope.dashboard.tabs && $scope.dashboard.tabs.length > 0) {
                $scope.dashboard.tabs[0].active = true;
            }
            $scope.page.loading = false;
// Demo for nijas
alert('User logged in after redirect to backoffice dashboard');    });

Screenshots: