1. You will need to set up a "New Custom Variable" in GTM. You can do so by:
going into "Tags" in the left-hand menu
selecting "New" to add a new tag
selecting "Custom HTML" as seen below
window._mfq = window._mfq || ;
window._mfq.push(["setVariable", "KEY", "VALUE"]);
Replace the "key" and the "value" to match your requirements, accordingly.
3. After that, you have to add a "Trigger".
You can trigger the tag on anything you want, but if for example, you want the tag to be set whenever a visitor clicks a specific button, you can do that by clicking inside the "Triggering card" -> and then clicking the "+" sign in the upper right-hand corner:
Click inside the new "trigger card" that appears and then choose Click > All Elements:
Once you've chosen the trigger type you can set it up like this:
This will make it trigger on a specific element on your site. In this example, you define the element you want the tag to trigger on by its CSS selector.
Finding the CSS Selector:
Finding the selector of an element in Chrome only takes a couple clicks using the built-in Developer Tools.
To find the CSS selector of a page element:
1. Right-click the element in the page and choose "Inspect" - The Developer Tools window will open.
2. In the Elements tab of Developer Tools, right-click the highlighted element and select Copy > Copy selector.
That's it! The selector string is now in your clipboard and you can paste it into your trigger.
4. When you've configured the trigger as shown above please "save" the trigger and the tag - and then click the "submit" button in the top right-hand corner.