I don't care why the OP wants it. He wants it, his system, his responsibility.
I do have a (somehow rhetorical) "question". The instructions and the GUI are so direct that I don't even see the point of the original question. Moreover, testing the functionality of the "custom" setting is so easy and fast, that it takes less time than this topic.
The technical answer to the technical question (and by this I don't mean that I am recommending using v4.8, unless it's Win 9x/ME) is that setting custom settings requires the "custom" selection (which is why it will automatically move to "custom" if you set customized settings).