What is server preview header

In the setup of the GTMs via the Stap tutorials there are

Web container: server preview header, transoprt_url, GA4 ID, Google Analytics Server-Side, Facebook pixel

What is the Server Preview Header Value?

I presume that the Transport_URL will match the subdomain created as recommended?

The server preview header no longer needs to be specified, it is taken automatically after a recent update.

Transport URL - yes, this is the link of your server container. If you have created a subdomain and configured it, you need to specify it.