Application/Access Group Selector

On Navigation Panel

If you are planning to display the Application Selector in the navigation panel in your classic portal, simply select "Show Application Selector"  option on Options tab of the portal rule. 

On Property Field

It also possible to have a Property field with AppSwitch Control. The property name has to be  pyPortal.pyCurrentApplication in order to show the current application.

On Top Menu Bar 

Add a MenuBar Control to your portal's top header, and define a Navigation rule  where you can Run Script. The details are as follows. 
* Remember to include  the required JavaScript file to the harness.