Custom Select All Report Parameter

Hello, using v7.1.1. I've got a report in the portal and the parameters are department and cost center. When I select a department it scopes the cost centers to just cost centers in that department. My problem is that there are a huge number of cost centers and I want a way to Select All cost centers. I'm using Web Designer to implement the functionality. I've got the button created and appearing in the right place on the Cost Center selection page but I'm a little lost as to what Server Action the button should implement. Can someone point me in the right direction as to what server action to execute and perhaps what collections to look at and update to select all cost centers.