try this
Private Sub cmbCustomerName_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmbCustomerName.Leave
cmbPlateNo.ValueMember = "car_id"
cmbPlateNo.DisplayMember = "plate_no"
cmbPlateNo.DataSource = dt.Tables(0)
cmbPlateNo.SelectedIndex = -1
end sub