You can use either a character format or a paragraph format to create a hotspot for a jump from a topic to a secondary window. Assign the window name to the hotspot format:
[SecWindows] ; doc format = name of secondary window to use for jumps from ; within the span marked by this format (same as WinHelp usage) HotspotFmt=wndwname
If more than one CHM file is involved in a jump, you must also specify how those files are mapped; see §18.14.1 Interlinking multiple CHM files.