You can use querystrings for sending selected index of dropdown and when you redirect just look for the value in Querysting if you get the value just select the value of dropdown.
just a snippet for your reference:
protected void btnSubmitProductFinder_Click(object sender, ImageClickEventArgs e)
{
var MakerId = drpVehicleMaker.SelectedItem.Value;
var strUrl = "";
strUrl = "productfinder.aspx?MakerId=" + MakerId ;
Response.Redirect(strUrl);
}
}
and on your pageload:
if (Request.QueryString["MakerId"] != null)
{
try
{
int drpVehicleMakerId = int.Parse(Request.QueryString["MakerId"].ToString());
drpVehicleMaker.Items.FindByValue(drpVehicleMakerId.ToString()).Selected = true;
let me know if it works.