The Task and Event reminder pop-up is enabled by default in all organizations. Administrators can disable it in Activity Settings under Setup.
Turn on pop-up reminders for Tasks and Events
1. Navigate to Setup and enter Activity Settings in the Quick Find box.
2. Click Activity Settings
3. Select Enable Activity Reminders
4. Click Submit.
Note: To receive Reminder pop-up notifications, your pop-up blocker must be disabled.
Turn off Pop-Up Reminders for Tasks and Events
1. Navigate to Setup and enter Activity Settings in the Quick Find box.
2. Click Activity Settings.
3. Deselect Enable Activity Reminders.
4. Click Submit.
Notes:
- This is applicable to both Salesforce Classic and Lightning Experience.
- Activity reminder pop-ups will not display if the only page open in the org was created with Visualforce (*.visual.force.com).