10 ILCS 5/20-2
Any member of the United States Service, otherwise qualified to vote, who expects in the course of his duties to be absent from the county in which he resides on the day of holding any election may make application for a vote by mail ballot to the election authority having jurisdiction over his precinct of residence on the official postcard or on a form furnished by the election authority as prescribed by Section 20-3 of this Article not less than 10 days before the election. A request pursuant to this Section shall entitle the applicant to a vote by mail ballot for every election in one calendar year. The original application for ballot shall be kept in the office of the election authority for one year as authorization to send a ballot to the voter for each election to be held within that calendar year. A certified copy of such application for ballot shall be sent each election with the vote by mail ballot to the election authority's central ballot counting location to be used in lieu of the original application for ballot. No registration shall be required in order to vote pursuant to this Section.
Ballots under this Section shall be mailed by the election authority in the manner prescribed by Section 20-5 of this Article and not otherwise. Ballots voted under this Section must be returned postmarked no later than election day and received for counting at the central ballot counting location of the election authority during the period for counting provisional ballots, the last day of which is the 14th day following election day.
10 ILCS 5/20-2