Button on Command bar to select multiple records and click on button to call JavaScript

I need to add a button on command bar so that when I select one or multiple records, I want to click on this button and call a web resource.

Web resource will do some magic. I have no problem in adding this button using Ribbon Workbench on a form however I can't work out how to add button on this command bar as you can see in above screenshot.

Any help appreciated.

Thanks