RSUs are taxed as salary the day they vest. Your employer usually withholds a flat 22% — which is not enough if you earn well. Then your broker reports a cost basis of zero when you sell, so the IRS taxes the same money again. This works out both.
From your vest confirmation. Use the shares that vested, before any were sold to cover tax.
Leave blank if you are still holding — the vest tax applies either way.
General guidance only, not tax or legal advice. Covers US-resident taxation of RSUs settled in shares. It does not model FICA on the vest, the 3.8% net investment income tax, AMT, section 83(b) elections, multiple vest tranches in one calculation, dual-status or nonresident years, or Indian tax where you are also resident there. Nothing you type leaves this browser.