[FDelegateHandle](API\Runtime\Core\Delegates\FDelegateHandle) InternalRegisterCustomEventHandler ( const [FName](API\Runtime\Core\UObject\FName) & EventMessageType, const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [IConcertSessionCustomEventHandler](API\Plugins\Concert\IConcertSessionCustomEventHandle-) > & Handler )