For WinHelp, assign the name of a secondary window to a hotspot paragraph or character format in [HelpWindowStyles], and assign the Window property to the same format in [HelpStyles]; for example:
[HelpStyles] JumpToExtra = JumpHot Green Window
[HelpWindowStyles] JumpToExtra = extra
See §17.8.6 Specifying jumps to secondary windows in WinHelp.