How to Set an Alarm on MacOS Using Reminders

Step 1: From the Dock, select the Calendar app. And select the date for which you would like to add an alarm.

Step 2: Double-click the space corresponding to the time of day you want to add the alarm.
For example, if you want an alarm for 3:30, select the space between 3 p.m. and 4 p.m. And enter a name for the event or reminder.

Step 3: Select Add Alert.

Step 4: Select the Alert drop-down menu and choose when you want to be reminded.

Credit : https://www.lifewire.com/how-to-set-alarm-on-mac-4690934