10 ILCS 5/20-2.1
Citizens of the United States temporarily residing outside the territorial limits of the United States who are not registered but otherwise qualified to vote and who expect to be absent from their county of residence during the periods of voter registration provided for in Articles 4, 5 or 6 of this Code and on the day of holding any election, may make simultaneous application to the election authority having jurisdiction over their precinct of residence for registration by mail and vote by mail ballot not less than 30 days before the election. Such application may be made on the official postcard or on a form furnished by the election authority as prescribed by Section 20-3 of this Article or by facsimile or electronic transmission. 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.
Registration shall be required in order to vote pursuant to this Section. However, if the election authority receives one of such applications after 30 days but not less than 10 days before a Federal election, said applicant shall be sent a ballot containing the Federal offices only and registration for that election shall be waived.
Ballots under this Section shall be delivered by the election authority in the manner prescribed by Section 20-5 of this Article in person, by mail, or, if requested by the applicant and the election authority has the capability, by facsimile transmission or by electronic transmission.
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 5/20-2.1