Add a button to the form with code along these lines in its Click event
procedure.  For this example I've assumed the list box, lstEmployees, is of
employees and the list box's bound column is a numeric EmployeeID:

   Dim varItem As Variant
   Dim strEmployeeIDList As String
   Dim strCriteria As String
   Dim ctrl As Control
   
   Set ctrl = Me.lstEmployees
   
   If ctrl.ItemsSelected.Count > 0 Then
       For Each varItem In ctrl.ItemsSelected
           strEmployeeIDList = strEmployeeIDList & "," &
ctrl.ItemData(varItem)
       Next varItem
       
       ' remove leading comma
       strEmployeeIDList = Mid(strEmployeeIDList, 2)

       strCriteria = "[EmployeeID] In(" & strEmployeeIDList & ")"
       DoCmd.OpenForm "frmMain", WhereCondition:=strCriteria
   Else
       MsgBox "No Employees Selected", vbInformation, "Warning"
   End If

Ken Sheridan
Stafford, England