Employers usually withhold bonuses at a flat 22% federal rate (37% over $1M), plus state tax and FICA.
Bonuses are supplemental wages: flat 22% federal withholding up to $1M, plus state tax and FICA. Your actual tax at filing time follows your bracket.
Flat 22% withholding plus state/FICA often takes 30–40% upfront; you may get some back at tax time.
You can adjust your W-4, but the 22% supplemental rate generally still applies at payment.