Hi
The sales order as a field is filled in BSEG only in case of customer down payments unless you are using a substitution or an enhancement to fill it in at the time of customer billing. Again, it may create an issue if you are doing billing against multiple sales order.
That said, if you are filling in the Sales Order at the time of billing, you can do that in T Code O7Z4 so that it appears in F-28 selection parameters.
regards
sanil