How can I filter directly on applications in Dimenssion.
Exactly what are you looking for?
There are scheduled reports on application usage, which may suit your needs.
Look at the Application Usage report section, here:https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/dimension/reports_report-list_d.html
Note that you need to do the correct logging, including having Application Control selected on your policies, in order to have data to populate the reports.