Modules & VBA :: Update Select List Coding?

The coding below works fine. It presents a form with a list box of counties. Allows the user to select ALL, one or several counties and returns a query containing the clients from those counties.

The fields showing in the query are First, Last, Add1, FLAGToMap, City, Prov and Sector_Name.

I want to add in there a choice to select only the records that have are TRUE (-1) in the FLAGToMap field - just like the ALL button, this would be an ALL Selected Button let's say.

I would not know where to begin as I copied and adapted the coding below from a sample database and don't understand - at all - how the query is generated. The only coding in the form is the one below.

Private Sub cmdOpenQuery_Click()
On Error GoTo Err_cmdOpenQuery_Click
Dim MyDB As DAO.Database
Dim qdef As DAO.QueryDef
Dim i As Integer
Dim strSQL As String

[code]....