Enter the amount of the wash loss (as a positive) in the box called "Wash Loss Disallowed." That will automatically adjust the gain or loss to disallow the wash amount and put a "W" in the correct column of the 8949.

IE, if the sales price is $100 and the cost is $50, enter those. The gain will show as $50. Enter a $1 in the wash box and the gain will change to $51.