Ill. Admin. Code tit. 26, § 150.25
The complainant shall serve a copy of the complaint upon the respondent. Service shall be complete when the document is served as provided in the Civil Practice Law [735 ILCS 5/2-203(a)], in person upon the party or his attorney, or deposited for mailing with the United States Postal Service, postage prepaid, registered or certified, addressed to the party.