Add GTM in order confirmation page

[Host-Age.ro]

Hello!

I want to add in </head> some custom code from Google Tag Manager (GTM) in the last page of order, confirmation page /cart/&step=5
But how? I saw I have only .tpl files...

Anybody can help me please?

Many thanks!

[tallship]

It's not real active around here nowadays, but with a bit of patience, someone should be along shortly to lend a hand