To get to the Google Tag Manager settings, click your user icon at the top right and navigate to "Team settings". There, an option to configure Google Tag Manager is available. Click "Configure" to get started:
In the pop-up that appears, you can enter the container ID in the format GTM-ABC123:
Then click "Enable Google Tag Manager" to enable the feature. In your account settings overview, you will then see the container ID you entered with a checkmark to indicate it's enabled:
From then on, newly created online reports and dashboards will have the Google Tag Manager container.
You can now head back to Google Tag Manager and add your snippets to your container. For example, you could add a Google Analytics snippet to see how often your reports are read, or add a snippet from your support system (Zendesk, Intercom, etc.) to start interacting with your clients directly from the report!