As far as I see neither this property nor HTMHelpTopic are available for newly created objects in Application Documentation branch of AOT even if I set HTMLFileEnabled to Yes.
The only cure is to restart the client.
Moreover, after that if you first choose anything but your newly created object in Application Documentation branch (if you e.g. search for it by typing its first letters of the name) or the current element you are working with in the branch loses the focus that is the case - you now cannot change these properties.
Correct me, please, if I missed something.