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, select the correct website project, and follow these steps:
Step 1 - Add a new tag
Click on the "New Tag" button:
In "Tag Configuration" select the “Custom HTML Tag” product:
Step 2 - Paste the code
Visit Account Settings to obtain your custom website tracking code.
Go to Settings → Tracking code and copy the code.
IMPORTANT: The code above is just an example, each tracking code is unique for everyone's account, so make sure you copy the one from your account.
In the “Tag Configuration” card - paste your tracking code into the “Custom HTML” text box:
Step 3 - Set triggers
Within the "Triggering" card - choose “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.