How to get alarm in my html custom widget

Please have a look here: Cumulocity IoT Web Development Tutorial - Part 3: Query data from Cumulocity IoT

The c8y/client comes with an AlarmService:
http://resources.cumulocity.com/documentation/websdk/client/classes/alarmservice.html

So you can easily query any data (also alarms) with it and visualize that data accordingly.