C# Authentification Form via PageMethods - ajax

Question

I have a problem regarding those tutorials :
https://msdn.microsoft.com/en-us/library/xdt4thhy%28v=vs.140%29.aspx
I ve create my own site with those tips, so on my HTML file, I ve of course insert inside the form :
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true"></asp:ScriptManager>
I've create a Logon.aspx page in order to put the login form, and by click, the button call a javascript function login():
<script type="text/javascript">
function login() {
var user = $('#username').val();
var password = $('#password').val();
PageMethods.login_GO(user, password, ok, ko);
function ok(result) {
alert(result);
}
function ko(result) {
alert('Cannot process your request at the moment, please try later.');
}
}
</script>
then the javascript function call a behind code method (inside the Logon.aspx.cs file)
[WebMethod]
public string login_GO(string user, string password)
{
if (user == "cool" && password == "whau")
{
FormsAuthentication.RedirectFromLoginPage(user, true);
return "done";
}
else
{
return "KO";
}
}
The only answer I have is an alert messagebox from the browser:
Cannot process your request at the moment, please try later.
Why do you think this method doesnt work ?
I mean normally I dont need anything else isnt ?
Thanks in advance !
EDIT:
Before I started to debug the code, I decide to disable the Authentication feature in the Web.config configuration (I only put a comment on all those lines below) :
<authentication mode="Forms">
<forms loginUrl="Logon.aspx" name=".ASPXFORMSAUTH" defaultUrl="PlazaMonitor.aspx" >
</forms>
</authentication>
<authorization>
<deny users="?" />
</authorization>-->
So now, I can call the login_GO method on the behind code (Logon.aspx.cs).
Every works fine, but not the Authentication (because I disabled it)
The goal of all of this, is to authenticate an user using a behind code method, so if I need to be Authenticated before I use the Authentication Method I m going to be crazy ^^ !
EDIT :
I just understand, EVERY ressource should be disabled until you are AUTHENTICATE, so even the WebMethod used to be authenticate is disabled !
Do you know where (in Web.config certainely) we could change/allow the good ressources we need ?
Thanks in advance for your help ;) !