To add Google Tag Manager to your website you'll need an account, as well as a container-ID. If you don't already have those you can follow this guide by Google to create them.
To add Tag Manager through the editor you don't need the installation code, you'll only need the container-ID, which can be found in the top right corner, or will be provided during container creation.
Select App Integrations
And then select Third-Party Integrations
From here click to add an integration and find Google Tag Manager in the list:
You should now see a field where you can enter your container-ID:
Add and click save. Click the toggle switch to activate your integration.
Click publish. Once publishing is complete Google Tag Manager will be active on your website.