How to set navigateurl for radwindow in javascript ?

I want to use a radwindow in my page, and I use it in tow part.

I have a radwindow that I use it in 2 part.

 <telerik:RadWindowManager ID=RadWindowManager1 ShowContentDuringLoad=false

              VisibleStatusbar=false  

         RegisterWithScriptManager=True 

        EnableShadow=True ReloadOnShow=true Width=800px Height=550px 

      runat=server>

      <Windows>

            <telerik:RadWindow ID=modalPopup runat=server Modal=True

      >

      </telerik:RadWindow>

 </Windows></telerik:RadWindowManager>

I use javascript for show radwindow.

<telerik:RadCodeBlock runat=server ID=rdbScripts>

      <script type=text/javascript>



          function showDialogInitially() {

              var wnd = $find(<%=modalPopup.ClientID %>);

              wnd.show();

              Sys.Application.remove_load(showDialogInitially);

          }

When I click on a button , set Navigateurl of radwindow = (DefCall.aspx) and when I click to other button , set navigateurl = (DefTask.aspx) and passe 2 value to the child page by QueryString or other way.

protected void btnDefCall_Click(object sender, EventArgs e)

{

        string strURL = string.Format(../PhoneCall/DefPhoneCall.aspx

    ?FormTypeID={0}&FormID={1}, number1,number2);

  modalPopup.NavigateUrl = strURL;



    ScriptManager.RegisterStartupScript(Page, GetType(), PopupScript, string.Format(javascript:showDialogInitially()), true);



}





 protected void btnDefTask_Click(object sender, EventArgs e)

{

    string strURL = string.Format(../Task/DefTask.aspx?FormTypeID={0}&FormID={1}, (int)clsHelper.FormType.Lead, int.Parse(Request.QueryString[ID].ToString()));

 modalPopup.NavigateUrl = strURL;



    ScriptManager.RegisterStartupScript(Page, GetType(), PopupScript, string.Format(javascript:showDialogInitially()), true);

}