The territory adjoining a metropolitan park district may be annexed to and become a part thereof upon petition and an election held pursuant thereto. The petition shall define the territory proposed to be annexed and must be signed by twenty-five registered voters, resident within the territory proposed to be annexed, unless the territory is within the limits of another city when it must be signed by twenty percent of the registered voters residing within the territory proposed to be annexed. The petition must be addressed to the board of park commissioners requesting that the question be submitted to the legal voters of the territory proposed to be annexed, whether they will be annexed and become a part of the park district.