Validating input data with BreezeJS / KnockoutJS / Durandal - asp.net

Question

I'm trying to validate my inputs from a form with BreezeJS / KnockoutJS / Durandal and I'm not getting any results.
This is my input inside my view:
<input data-bind="value:app_name, attr: {placeholder: resources.brand_create_placeholder_brand_name }" type="text" class="form-control input-sm" name="app_name" id="app_name" />
And this is my viewmodel:
define(['durandal/app', 'services/logger', 'plugins/router', 'knockout', 'breeze', 'toastr', 'services/resources'],
function (app, logger, router, ko, breeze, toastr, resourcesSVC) {
var vm = {
app_name: ko.observable(), //.extend({ required: true, minLength: 30, }),
...,
currency: ko.observable(),
currencies: [{ name: "Euro", value: "EUR", synbol: "€" }, { name: "Dollar", value: "USD", synbol: "$" }, { name: "Pounds Sterling", value: "GBP", synbol: "£" }],
use_smtp: ko.observable(false),
activate: activate,
title: "Create Brand",
createEntity: createEntity,
goBack: goBack,
resources: {}
}
};
Here I'm initializing my service:
var serviceName = 'breeze';
var manager = new breeze.EntityManager(serviceName);
And here I have the creation of my entity:
function createEntity() {
var DT = breeze.DataType;
var Validator = breeze.Validator;
var brand = manager.metadataStore.getEntityType("app");
/*app_name*/
var propAppName = brand.getProperty('app_name');
brand.addValidator(Validator.required(), propAppName);
brand.addValidator(Validator.maxLength({ maxLength: 30 }), propAppName);
var newBrand = brand.createEntity('app');
newBrand.app_name = vm.app_name();
...
manager.addEntity(newBrand);
manager.saveChanges()
.then(createSucceeded)
.fail(createFailed);
function createSucceeded(data) {
toastr.success("Brand created");
app.trigger('brand:changed');
router.navigate('#brands');
}
function createFailed(error) {
var msgError = "Create failed: " + getErrorMessages(error);
toastr.error(msgError);
manager.rejectChanges();
}
}
Can you please help me set validators for each field & display a custom message for each one whose values aren't correct?
Thank you!