Employers can register electronically through the MyTax Illinois​ website. You can register electronically using MyTax Illinois, available at mytax.illinois.gov.At the end of the day, the most important forms that you are going to have to submit include the I-9, W-4, IL-W-4, and W-9 forms. How to fill out the Illinois Employment Security Registration Form? 1. Gather your business and contact information. 2. Register as a new employer using the New Business Registration application (Form REG-1) on MyTax Illinois. All US employers must properly complete Form I9 for every individual they hire for employment in the United States. This includes citizens and noncitizens. How to fill out the Illinois Employee Registration Card Renewal Form? Head to the Illinois Department of Revenue (DOR) website.