We're talking about Git here. The question is not "how VSCode is programmed", the question is "does Git have a special field for commit trailers". The answer is no. Git stores the trailer as part of the commit message.
If you look at the comment I’m responding to, it is in fact about how VSCode is programmed; specifically, a possible reason why the Co-Authored-By trailer doesn’t show up in VSCode’s commit message box.