...
In your GTM account create a Tag using Custom HTML
Paste in your code from above
It looks like this:
Now add a Trigger that fires this Tag on your checkout confirmation page (note this must fire after your GA purchase event), so an easy way to do this is to use the purchase event as the trigger. Follow these steps:
Create a purchase Trigger that uses a Custom Event of
purchase
It looks like this:
Hit Save
No Now use the GTM preview feature to make a test transaction and see your Tag firing.
...