The commissions would be deducted from the Sales Proceeds. In most cases the amount reported has already been reduced by any fees or commissions.
However, if you need to report fees, commission, or other sales expenses that were not subtracted from the proceeds reported on Form 1099-B, you can enter these expenses by selecting 'This sale involves an employee stock plan (including ESPP) or an uncommon situation' for this transaction.
See the screenshot below.
