C# - Post Value To New Window From Gridview Selected Column?

I tried to post the selected ID number from source page into a new target page(pop up window) and display more details on that page.

I have googled the varies solutions but seem like they are a bit out of date.
Here are my questions:

What's the best way to post it (session? cookie?) or Page.PreviousPage.FindControl? or add a control into <ItemTemplate>? or Javascript?

I don't want to use a query string since I don't want the user to manually modify it.

What should I write on the target page to grab that value?

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
string testID= GridView1.SelectedRow.Cells[3].Text;
// eg. Session["ID"] = testID;
//eg. open new pop up window ("target.aspx");
}