protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    if ( Page.IsPostBack )
    {
        ServerFormatSSN();
        IsValid=bIsValid;
    }
    if ( Page.ClientTarget.ToLower()!="downlevel" )
    {
    Page.RegisterClientScriptBlock(
     "FormatSSNClientScript", "<" + 
     "SCRIPT Language=JavaScript SRC=\"FormatSSN.js\"></" + "SCRIPT>");
    }
}

Example 3: OnLoad method of FormatSSN.