So when adjusting the cost basis, do I go with what's on the supplemental or what's in box 14 of W-2? Basically the question comes from having sold short term all the RSU's identified as ordinary income on the W-2 but somehow the brokerage generated supplemental form does not show that full amount on the adjusted cost basis. Not sure why. But of course, withholding was based on the full vested amount noted on the W-2. From what I've been seeing, it looks like I could adjust the cost basis to what's reported on the W-2 which generates a bigger capital loss than using the adjusted cost basis on the supplemental. Any thoughts?
Your employer issued Form 3922. Use that form to find your RSU cost basis.
I think 3922 is for ESPP's not for RSU's granted as ordinary income. So the question remains about the final source of the true cost basis.
Your Cost Basis for selling your RSU shares would be your vesting price (which could be shown on form 3922), plus the amount you were taxed as ordinary income on your W-2, divided by number of shares sold, as reported on a 1099-B. If you know your correct cost basis, you can enter as a regular stock sale in TurboTax.
Here's detailed info on RSU's and Taxes and How to Report RSUs.