Mo. Rev. Stat. § 256.649
Counties listed in section 256.643 which do not join the district pursuant to the provisions of sections 256.645 and 256.647 may subsequently join the district if the governing body of such county, after obtaining approval from the district commission, adopts an order declaring that the county shall join the district and establishing the precise boundary of the district within the county and if, after submitting the question of joining the district to the qualified voters of the county to be included in the district, a majority of such votes cast on the question favor joining the district. Such vote shall be conducted at an organizational meeting conducted pursuant to subsection 1 of section 256.647. Following the approval of the question favoring joining the district, the qualified voters of the county shall elect one commissioner to the commission to serve a term of office of six years as provided in subdivision (2) of subsection 1 of section 256.647. Candidates for election to the commission shall be citizens of the United States, voters within the county for one year next preceding the election, and at least thirty years of age.
§ 256.649, RSMo