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();
}