Hi Martin,
First you need to add a global mbox to the mbox configuration on the T&T side. Login and go to Configuration > mbox > edit and add the following to the "extra javascript box":
/* global mbox */
mboxCreate('global','path='+location.pathname);
Note: the parameter for location.pathname is to hep to populate the page URL so it is easier to know where the mbox is when using Tagmanager.
Next, create a container in Tag Manager and add the product Adobe Target using your clientcode, which you can get from clientcare. Click on "synchronous" and click on "Save and Deploy".
This should be all you need to use TagManager for Test&Target.
Thanks,
Spring
If you find an answer helpful, mark it with a 'heart', if it answered your question then mark it 'correct'.