Use this tool to estimate the federal income tax you want your employer to withhold from your paycheck. This is tax withholding.Employers generally must withhold federal income tax from employees' wages. To figure out how much tax to withhold, use the employee's Form W-4. You must file Form IL-W-4 when Illinois. Income Tax is required to be withheld from compensation that you receive as an em- ployee. Request to withhold taxes. Submit a request to pay taxes on your Social Security benefit throughout the year instead of paying a large bill at tax time. Use the following pages to complete your Illinois State and Federal income tax withholding, or W-4 information.