Hi Juliya
Sales order field is not available on the FI side, so you will not find the same in F-28 Additional selection.
Alternatively, what you can do is to use the reference number or document type that are specific to your sales transactions. Populate the sales order number or billing document number in the reference field of the billing related financial document and provide the same in F-28 Additional selection -> Reference field for clearing purpose.
Similarly next closest option that I would think is to go with Document type that is specific for billing.(Doesnt suit in all cases).
Thanks,
Sekhar