http://www.sqlservercentral.com/blogs/data-adventures/2014/07/22/ssrs-report-subscription-fire-event-from-ssis-package/
Quick and dirty way is to execute dbo.AddEvent stored procedure in the SSRS database and provide two parameters: @EventType='TimedSubscription', @EventData=@Subsciption_ID (primary key from the dbo.Subscriptions table)
You can also use the SSRS web service; see link above.
