Sorry that seems a little confusing to me. Are you saying that windows doesn't support F13-24?
Sorry, what I mean is we use a standard Windows control for entering hotkeys. Based on a customer report before, it seems this specific control isn't aware of those keys. Possibly an oversight by Microsoft.
Because of this Multiplicity cannot accept those as hotkeys. If you know the scancodes of those keys it might be possible to directly edit the registry to create a hotkey entry however.
The other thing to try is if shift plus say f11 triggers when you press f13 as there are suggestions that might be a mapping used by Windows sometimes.