Private Sub ComboBox1_KeyDown( _
  ByVal KeyCode As MSForms.ReturnInteger, _
  ByVal Shift As Integer)
    Dim intI As Integer
    Dim strValue As String
    Dim blnFound As Boolean

    If KeyCode =  13  Then
        strValue = ComboBox1
        
        For intI =  0  To ComboBox1.ListCount -  1 
            If ComboBox1.List(intI) = strValue Then
                blnFound = True
                Exit For
            End If
        Next intI
        
        If Not blnFound Then
            Worksheets("Лист1").Range("A1"). _
              Offset(ComboBox1.ListCount,  0 ) = strValue
            ComboBox1.RowSource = "Лист1!A1:A" & ComboBox1.ListCount +  1 
        End If
    End If
End Sub