You can
HookEntityOutput (for every entity of the same class):
Code:
HookEntityOutput( "func_button", "OnPressed", MyCallback ); // hooks "OnPressed" output for every entity with "func_button" classname
or
HookSingleEntityOutput (for specific single entity):
Code:
HookSingleEntityOutput( iEntity, "OnPressed", MyCallback, true ); // hooks specific iEntity (index) and calls MyCallback only once (last boolean argument) when entity fires "OnPressed" output
Outputs for func_button.