Appends the event handler represented by the specified delegate interface to the set of event handlers for the current EventSource object.
_In_ TDelegateInterface* delegateInterface,
_Out_ EventRegistrationToken* token
The interface to a delegate object, which represents an event handler.
When this operation completes, a handle that represents the event. Use this token as the parameter to the Remove() method to discard the event handler.
S_OK if successful; otherwise, an HRESULT that indicates the error.