Homebrew Creation Hyperlinking

What I mean by hyperlinking: There are little pop-up windows of spell/ monster stats etc., already used across the site in a variety of ways, which you can create yourself with something like [spell]Example Spell[/spell] when homebrewing. I've discovered you can only do this with existing official things, as opposed to homebrew ones. I'd like to be able to do this with homebrew things.