Use the DMax function to get the ID of the supplier you just entered (it will be at the end of the table) Then set the Combo Box equal to that ID.
You could do something like this:

Dim ID as Long

ID = DMax("SupplierID", "SupplierT", "SupplierID=" & ID)
cboSupplier = ID

Hi Kevin,  Thanks so much for this - I would never have thought of using DMax on the underlying table to get the just entered data back.  I don't need the criteria parameter in the DMax statement (I don't think it would work?) and I need to requery the combo box before executing the 'cboSupplier = ID' line but this works fine!  I was using tempVars to remember the data entered on the popup form before closing it, but then didn't know how to assign the stored values to the combo box.  That's because the cboSupplier line escaped me as well - I don't find it intuitive.  When a value is assigned to the combo box like that, is it assigned to a particular column, or does it (as it seems to) somehow refer to a record primary key in the underlying table?

I use the SQL statement:

SELECT 0 AS SupplierID, "(Add New Supplier)" AS SupplierName, "A" AS SortOrder FROM tblSuppliers UNION SELECT SupplierID, SupplierName, "B" AS SortOrder FROM tblSuppliers WHERE SupplierName IS NOT NULL ORDER BY SortOrder, SupplierName

as the combo box row source.  That gives me a sorted list of Supplier Names with '(Add New Supplier)' at the top of the list.  When the user chooses an item from the list I use the combo box OnUpdate event to see if (Add New Supplier) was chosen and if it was I open up the secondary form.