Your employee must complete Form IL-W-4, Employee's Illinois. Withholding Allowance Certificate.Have each of your employees fill out a Form ILW4. Register with the state using the MyTax Illinois portal. 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. Use the following pages to complete your Illinois State and Federal income tax withholding, or W-4 information. The legislation created a uniform 30day threshold for a nonresident to be subject to Illinois income tax. Effective for 2020, nonresident employees that travel and work in Illinois for 30 work days or less in a calendar year are not subject to Illinois income tax. Generally not considered a resident for Illinois income tax purposes.