Seen here.

function AddToCalendar(calendar,server,eid,date){
  var url=server+'/calendar.ics?eid='+eid+'&calendar='+calendar;
  if(date) {
    url+='&date='+date;
  } else { 
  if(document.mgform&&document.mgform.selecteddate) {
    if(document.mgform.selecteddate.selectedIndex<1) { 
      alert('Please select the date you would like to attend.');return;
    } else { 
      url+='&date='+document.mgform.selecteddate[document.mgform.selecteddate.selectedIndex].value;}
    }
  }
  if (calendar=='outlook'||calendar=='ical')
    document.location.href=url;
  else
    window.open(url,'calendar','toolbar=yes, menubar=yes, location=yes, status=yes, scrollbars=yes,resizable=yes, width=800, height=600, left=0, top=0');
}

and

  <a href="javascript:AddToCalendar('google',%20'http://www.eventbrite.com',%20'547456457',%20'');">link</a>