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.