How to delete an event in vtiger using REST APIs and where to get WEBSERVICE_ID from?

I want to delete an event in Vtiger Calendar using REST APIs. I'm trying the following:

POST /webservice.php HTTP/1.1
operation=delete
sessionName=sessionId    // Obtained through Login Operation
id=<WEBSERVICE_ID>

but I don't know what WEBSERVICE_ID is and where to find it.

When I create an event using REST-API it returns an array with the following data:

{"subject":"Follow up Test","assigned_user_id":"49x1","date_start":"2016-06-17","time_start":"09:00:00","time_end":"18:00:00","due_date":"2016-06-17","recurringtype":"","parent_id":"","contact_id":"","taskstatus":"","eventstatus":"Planned","taskpriority":"High","sendnotification":"0","createdtime":"2016-06-23 11:18:20","modifiedtime":"2016-06-23 11:18:20","activitytype":"Event","visibility":"","duration_hours":"9","duration_minutes":"0","location":"","notime":"0","modifiedby":"49x1","created_user_id":"49x1","source":"WEBSERVICE","starred":"0","tags":"","reminder_time":"","description":"Test Description","id":"1x5748","label":"Follow up Test"}

There is an id in the array ("id":"1x5748"), but when I try to use the id as a WEBSERVICE_ID in the delete operation I'm getting the following error:

Fatal error: Uncaught exception 'Exception' with message 'Error: Id specified is incorrect'