In your aspx page where object tag is present..Add init param tag of your own.
<object data="data:application/x-silverlight-2," ID="Silverlight2" type="application/x-silverlight-2" width="100%" height="100%">
<param name="source" value="ClientBin/ProductProDemo.xap"/>
<param name="onError" value="onSilverlightError" />
<param name="background" value="white" />
<param name="minRuntimeVersion" value="4.0.41108.0" />
<param name="autoUpgrade" value="true" />
<param name="initParams" value="startPage='<asp:Literal id="id" runat="server"/>'"></param>
<a href="http://go.microsoft.com/fwlink/?LinkID=149156&v=4.0.41108.0" style="text-decoration:none">
<img src="http://go.microsoft.com/fwlink/?LinkId=161376" alt="Get Microsoft Silverlight" style="border-style:none"/>
</a>
</object>
In the code behind file of the same aspx page ie aspx.cs file..
On page load provide the value to your init param
protected void Page_Load(object sender, EventArgs e)
{
id.Text = "Page1".ToString();
}
In your App.xaml.cs file you can access the value as
private void Application_Startup(object sender, StartupEventArgs e)
{
string startPage = e.InitParams["startPage"];
this.RootVisual = new MainPage();
}