How to reference the "Subtotal" cell in a mtraix report?

In a matrix report, column grouping, I added a "subtotal" to the column grouping by right-clicking on that cell and selected "Subtotal". Everything is working fine except when I tried to pass the parameter value to the drill-through report I need to set the parameter value if it is the "Subtotal" cell users click on it and I am not sure how to reference the "Subtotal' cell.

I have tried

=IIF(Fields!Cancer_Centre.Value="Subtotal",Parameters!DIMCancerCentreCancerCentre.Value,"[DIM_Cancer_Centre].[Cancer Centre].&[" & Fields!Cancer_Centre.Value & "]")

Thanks

I got it work afetr so many attempts. I wrote an expression to check the value if the cell is the "Subtotal" value or one of the value in the column grouping. Need to think outside the box to solve this.

Thanks.