Appending text to editor using Javascript

Hi Guys,

ANy help appreciated on this one as I'm really stuck but know the solution will be simple. Basically I have a javascript button on my page which will append text to the existing content in the CKEditor when clicked. Problem is I can't seem to get it to append. My code is as below

function addCaseStudiesTemplate() {
      var e = CKEDITOR.instances['ctl00_PageContent_oEdit1'];
      if(e){
            e.value = e.value & 'Replace with th';
      }
}