Note:

Remember that the New Event global action doesn’t support event reminders, even though you can add them to the global action layout.

Reference Help Article – https://help.salesforce.com/articleView?id=sf.events_enable_reminders.htm&type=5

Salesforce Idea – https://trailblazer.salesforce.com/ideaView?id=0873A000000cQrGQAU

1.  Enable Enable Activity Reminders.

2. Enable Enable in-app notifications.

3. Create a new Quick Action with Action Type “Create a Record”. Select Event as the Target Object.

4. Remove the Global New Event Quick action and Add the Quick Action to the page layout to test it.

Output:

Create Event record through Apex:

Event objEvent = new Event();
objEvent.Subject = 'Test 1';
objEvent.StartDateTime = System.now().addMinutes( 5 );
objEvent.EndDateTime = System.now().addMinutes( 35 );
objEvent.IsReminderSet = true;
objEvent.ReminderDateTime = System.now();
insert objEvent;