There is no globally set key in the ClassicWB packs - everything is set to default, so that must be F10 for most packs (although some seem to use print screen).
If you want to change universal settings, go to Settings (top left corner menu) ->Startup->WHDLoad. There you can alter global options.
The same settings can be found under the Launch menu->Settings->Startup or double clicking the WHDLoad.prefs file in the S directory.
To set the quit key individually, put the option in the tool types.