ifThenElse added into code below enables new records (customers) in main form

Private Sub Form_Current()
    ' clear listbox
    Dim X As Long
    Dim db As Database
    Dim rs As Recordset
          
    For X = 0 To ProductList.ListCount - 1
        ProductList.Selected(X) = False
    Next
    'loop through records for this customer
    Set db = CurrentDb

    'Null CustomerID on the form occurs when clicking through past last record to 'New' record
    '....so for 'New' record, SQL will load all from CustomerRecordT and then exit sub.
   If IsNull(Me.CustomerID) Then
            Set rs = db.OpenRecordset("SELECT * FROM CustomerInterestT")
            Exit Sub  'exit sub here if 'new' record otherwise WhileWend loop below will select items in listbox
    Else
        'SQL includes the WHERE statement when the active form has a CustomerID
        Set rs = db.OpenRecordset("SELECT * FROM CustomerInterestT WHERE CustomerID=" & CustomerID)
    End If

    'select the items in the list box that are included in the CustomerInterestT for the customer
    While Not rs.EOF
            For X = 0 To ProductList.ListCount - 1
                If CLng(ProductList.Column(0, X)) = rs!ProductID Then     'ListBoxes always return text i.e. "1"  
                    ProductList.Selected(X) = True
                End If
            Next
        rs.MoveNext
    Wend
    
    rs.Close
    db.Close
    Set rs = Nothing
    Set db = Nothing
End Sub