When you decide the user is setting the alarm, you will have to assemble the time for the alarm to happen.
Probably 2 digits for the hour and 2 digits for the minute.
Once the digits are entered, maybe look for a * as a terminator that defines the numbers are okay.
Maybe # would cancel setting the alarm as data entry is in error.
If after the 4th digit you enter more digits, display ERROR, or go back to the 'hours' entry.
Each time through loop, you will then compare the entered hours and minutes to the current hours and minutes.
After you get a compare the alarm is turned on/off/canceled.
Also, the TimeAlarm library allows you to make a 'one time alarm'.
See examples.
Alarm.alarmOnce(hours, minutes, seconds, function);