70 ILCS 3615/3.06
The initial territory of the Authority shall be Cook, DuPage, Kane, Lake, McHenry and Will Counties, Illinois. Any other county or portion thereof in Illinois contiguous to the metropolitan region may be annexed to the Authority on such conditions as the Authority shall by ordinance prescribe, by ordinance adopted by the county board of such county, and by approval by the Authority. Upon such annexation, a certificate of such action shall be filed by the Secretary of the Authority with the County Clerk of the county so annexing to the Authority and with the Secretary of State of Illinois and the State Department of Revenue.
No area may be annexed to the Authority except upon the approval of a majority of the electors of such area voting on the proposition so to annex, which proposition may be presented at any regular election as provided by the county board or boards of the county or counties in which the area in question is located. Such county board or boards shall cause certification of such proposition to be given in accordance with the general election law to the proper election officers who shall submit the proposition at an election in accordance with the general election law.
70 ILCS 3615/3.06