You are mixing up apples & oranges.... if the broker withheld federal or state taxes when you had the sales then you do NOT reduce your gain or your basis. https://ttlc.intuit.com/questions/2786576-where-do-i-enter-taxes-withheld-on-a-1099-b
And the form 8453 is used to send a copy of the broker statement if you choose to not enter every sale and only enter the category totals on the form 8949 ... see the last option ... https://www.irs.gov/pub/irs-pdf/f8453.pdf
