Adding the tracking code via Google Tag Manager follows the same process as adding any kind of HTML code snippet to your website.
Login to your GTM account and select the correct website project and follow these steps:
Step 1 - Add new tag
Click on the "New Tag" button:
In "Tag Configuration" select the “Custom HTML Tag” product:
Step 2 - Paste the code
Visit Settings to obtain your custom website tracking code
In the “Tag Configuration” card - paste your tracking code into the “Custom HTML” text box:
Step 3 - Set triggers
Within the "Triggering" card - chose “All Pages” to have the code run on all pages:
Note: In case you want the code to run on specific pages (e.g. landing pages for your protected campaigns) select the “Some Pages” option and define your own rule.