In short, you can't or you read the required information in a second step. Use the GetEventInformation service to obtain information about active event states. It returns all information required to acknowledge alarms.
Note, that the execution of GetAlarmSummary and GetEnrollmentSummary is deprecated, see future Addendum 135-2012av. Regardless, initiation will be still required if a device doesn't support AE-INFO-B.