When User Agent Info power-up is enabled, the following headers are added to the Incoming HTTP Request inside the sGTM:
To use User Agent Info power up, log in to your stape.io account, open the sGTM container, go to the Power Ups tab, and click Use User Agent Info power up. After that, enable the power-up and click Save.
Once done, open the server Google Tag Manager container preview, go to the Request tab, click on any incoming request, and expand the Request Headers section. You should see user agent info headers are added, similar to what you see on the screenshot below.
To use these Requests Headers in the server Google Tag Manager create a new variable with the variable type Request Header and type Header Name.