Adds an event handler to this event endpoint.
"keyboardEvent_var" is a variable referencing a KeyboardEvent object. | 
        
"keyboardEvent_var" is a variable referencing a KeyboardEvent object. | 
        
| Type | Description | 
| boolean | Returns true if the handler was successfully added to the set of event handlers. | 
| Name | Type | Description | 
| handler | KeyboardEventHandler | The client implemented KeyboardEventHandler to be called when this event is triggered. |