Access the module Portal of your website through the following link:
On the module Portal, select the website in which you wish to set the GTM (usually, it is “Default”).
On the tab Checkout, right after the selection of the website, there is the domain Google Tag Manager in which you have to insert the informed ID.
The ID format will always be "GTM-XXXXX", being XXXXX the exclusive identification to your account.
Right after finishing setting the things presented above, save the settings.