title: How to add Google Tag Manager code in Prestashop without a module
body: <p>I use Prestashop version 1.6.1.24 for my website and I would like to add Google Tag Manager code in it. All the solutions I have found are either partial or they use a paid module.</p>
  <p>Is there a way to add GTM code in Prestashop without using a module ?</p>