Server side tracking

Hallo,

I am trying to set up a server side tracking and I need to use a sub tracking domain to send my request to the server side.

In order to do that, I need to connect my own sub domain to the GTM container.
I need to change the GTM container code as written in the STEP 9 in this tutorial (https://gtm-server.com/how-to-set-up-google-tag-manager-server-side-container/).

Where do I find the GTM container code?

Thanks for your help!

The page I need help with: [log in to see the link]