Show (Display) JavaScript Alert Message box in middle of screen from code behind in ASP.Net

Show (Display) JavaScript Alert Message box in middle of screen from code behind in ASP.Net

The pop-up is on top center of the page, please suggest for middle of the page.

protected void Page_Load(object sender, EventArgs e)
{
    string message = "Hello! Mudassar.";
    System.Text.StringBuilder sb = new System.Text.StringBuilder();
    sb.Append("<script type = 'text/javascript'>");
    sb.Append("window.onload=function(){");
    sb.Append("alert('");
    sb.Append(message);
    sb.Append("')};");
    sb.Append("</script>");
    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());
}