Forms :: Filter Based On Multiple Selections - Referencing A Control On A Subform

I have a multi select list box which I am trying to use to open another form and apply a filter based on the user's multiple selections.

List box - lstCatergories
Main Form - frmSuppliersSummaryCategories
Subform on Main Form - frmSuppliersSummaryCategoriesSubForm

The Subform contains the text box CategoryID

Here's my code which nearly works...

Code:
Private Sub cmdFilterSuppliers_Click()
On Error GoTo Err_cmdFilterSuppliers_Click

Dim strWhere As String
Dim ctl As Control
Dim varItem As Variant

[Code] .....

My problem is that the control "CategoryID" is on the subform and I'm having massive problems trying to reference it. This is the part of my code that won't work:

Code:
DoCmd.OpenForm "frmSuppliersSummaryCategories", acNormal, , "CategoryID IN(" & strWhere & ")"

However; if I open the SubForm directly it works perfectly. I.e.

Code:
DoCmd.OpenForm "frmSuppliersSummaryCategoriesSubform", acFormDS, , "CategoryID IN(" & strWhere & ")"

To summarise... How do I reference the control "CategoryID" on the subform "frmSuppliersSummaryCategoriesSubform"?