Odoo autogenerates the ID of the sale.order.line once it is saved.You can do something like this: (Server Action code adds a note line to a purchase order, but the same technique works for Sales Orders)

for record in records:
  record['order_line'] = [(0, 0, {'name': 'Note: Downpayment requested', 'product_qty': 0, 'display_type': 'line_note'})]