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.Form I-9 - Required. All new employees must complete an I-9 form. Have each of your employees fill out a Form ILW4. Register with the state using the MyTax Illinois portal. Complete Form W-4 so that your employer can withhold the correct federal income tax from your pay. Give Form W-4 to your employer.