I'm on desktop but believe Online works same way
If you are inputting a 'sales summary' there should be a checkbox or drop down for "I need to adjust my total cost basis" and you can input it there as code W.
If you are inputting as "one by one" there should be a checkbox or drop down for "I have other boxes on my 1099-B" and it will show more explicitly a Box 1g to fill in
If you don't have many wash sales you are better off entering them as "one by one" with the individual details which will generate Form 8949, and subtract these items from your sales summaries so your schedule D which combines everything, is whole. Same with any other adjustment codes e.g. Accrued Market Discount is common (Code D / Box 1f). Any of these type adjustments to the sale summary (also non-covered securities) will trigger the need to mail your 1099-B to IRS with the details that you can easily provide electronically on Form 8949.
