US supplemental withholding on this page is 22% of the bonus up to $1,000,000, then 37% of the amount above that. Social Security and Medicare are then calculated with the same employee rates as the paycheck page, including the wage base. A 401(k) percent, if you enter one, reduces the bonus before income withholding on this form and does not reduce those payroll taxes.
A $10,000 bonus under the wage base has $2,200 of supplemental withholding at 22%, plus employee Social Security and Medicare on the full $10,000. The amount deposited is the bonus minus those amounts. A bonus that pushes year-to-date wages over $184,500 stops additional Social Security.
This is US supplemental withholding, not the tax you ultimately owe, and not Canadian bonus tax. It is not a flat 22% of tax due on your return. The final tax depends on the rest of the year’s income.