var username_data = $scope.LoginUsername;
var password_data = $scope.LoginPassword;
var data = {username : username_data, password : password_data};

$http.post('/api/login', data)
.then(function (res) {
    //to do...
});

app.post('/api/login', function(req, res) {
    console.log(req.data.username);
    console.log(req.data.password);
});