Both the Submit Order and Submit Top-up Order endpoints have a description field that can be used to provide this information. It is an optional field.
You can also use it to filter your order by the Get Order List endpoint – filter for the customer ID or partner order reference, if you have provided it during the submission process.
Here is the documentation of the Get Order List:
Description: Optional. A string to filter orders by their description. This performs a like search using the format '%DESCRIPTION%'."
Go to the partner documentation site for more information on the Submit Order endpoint.