Set value of TextBox after function execution

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ASP.NET

hi everyone
I try to make a site with a asp.net web application, but I can't continue because, I can not set the value of TextBox to "finish" after the execution of a function file download.

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                TextBox1.Text = "start";
            }
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            Response.ContentType = "application/pdf";            
            Response.AppendHeader("Content-Disposition", "attachment; filename=print_ordre.pdf");
            Response.TransmitFile(Server.MapPath("~/print_ordre.pdf"));
            //Response.End();

            TextBox1.Text = "finish";
        }

I await your help ...