How to add a hook on Button Kind?
 

I have managed to add a hook on String Kind but don't seem to find how to add a hook on Button Kind. How to add a hook on a button to be triggered on button click? Is this even allowed? Or clicks on buttons are only to be handled as part of IDCMP_GADGETUP handler?

You mean by GTST_EditHook? This is an attribute of string gadgets. There is no such thing for button gadgets.

If you use BOOPSI instead of GadTools, you can use the ICA_TARGET attribute to get a custom class get its OM_UPDATE method called. A class is similar to a hook, you just have to call MakeClass to get the hook pointer and NewObject to get an instance of this class to be used with ICA_TARGET.

Thanks thomas. Will check BOOPSI as well.