I'm not sure it can be done manually. If you open the new post screen in the virtualisation section there is a drop down next to the title to choose a tag; Jails is one of the options. They vary based on the forum.
I didn't realize those thread title tags where predefined. I was under the impression they can be created with random text like the other tags in the tags box under the forums editor. Thanks for the info usdmatt.
On each forum we can define a list of 'labels', in the case of "Virtualization and Emulation" we made a list of the various emulators/virtualization platforms. You can use it to make it easier to identify the question's subject or platform.