which code you want to execute and your syntax is wrong
BtnSubmitSurveyInfo.Attributes.Add("onclick", " var aa=confirm('Confirm - Are you sure you want to go ahead.')if (aa==true){your code}else {your code}");
this would we also handy
[update]
ScriptManager.RegisterStartupScript(this.Page, this.Page.GetType(), "script", "<script type='text/javascript'>var x = window.confirm('Do you want to assign pages to this advertise?');if(x){code}</script>", false);
regards....:)