Sign in to Google Tag Manager.

You'll find the Google Analytics Tag in the list of account tags.



Edit the existing tag:



Add a new custom dimension





Then you have to edit the value of the new dimension. To do this, press "new variable".



In the window that appears, choose "Custom Javascript":




In the " Custom Javascript " box , copy the following code:


function () {
  var cookie = document.cookie;
  if (cookie.indexOf('fstrz=') !== -1) {
    return cookie.indexOf('fstrz=true') !== -1 ? 'yes' : 'no';
  }
  return 'untracked';
}



Then, after saving, you can finalize the creation of the custom variable as below:



Finally, all you have to do is save the tag update and then publish the tag.