setting a default slicer on drill down page based on field parameter

I have set up a page using field parameters to slice and display the data, the field parameter are either customer or product related.

I have also created a drill down page - where when the use selects one of the row headers in the table they can drill through to get the detail.  The detail page needs to display either by item or customer depending on the original field parameter chosen.

I have set up another field parameter on the drill through page but need to set a default for this on the drill down page based on whether it is a customer or item. How do i do this?