From the home screen of MyFrontpoint.com
Click on the Notifications sections on the menu options on the left side of the screen.
Click on the +New Notification button to create a new notification and select Sensor Left Open from the list. From here you can check the boxes next to any door or window sensors you would to receive notifications for and then the amount of time they need to be open before the notification is sent.
Click on +Add Recipient to select which phone numbers, emails or push devices you would like to receive the notification. You can also use the During this time frame: section to limit by times of day or days of the week that you will receive the notifications.
Click Save when done.