Hi @alucardg,
Welcome to the Ext.NET forums!
In this case you should submit a Window's Title manually via, for example, a DirectEvent's extra parameter.
Example
<%@ Page Language="C#" %>
<script runat="server">
protected void GetTitle(object sender, DirectEventArgs e)
{
X.Msg.Alert("GetTitle", e.ExtraParams["title"]).Show();
}
</script>
<!DOCTYPE html>
<html>
<head runat="server">
<title>Ext.NET v2 Example</title>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:Window ID="Window1" runat="server" Title="Initial Title" />
<ext:Button runat="server" Text="Get title">
<DirectEvents>
<Click OnEvent="GetTitle">
<ExtraParams>
<ext:Parameter Name="title" Value="App.Window1.title" Mode="Raw" />
</ExtraParams>
</Click>
</DirectEvents>
</ext:Button>
<ext:Button runat="server" Text="Set title" Handler="App.Window1.setTitle('New Title');" />
</form>
</body>
</html>