Mo. Rev. Stat. § 67.1721
In the event that the proposed metropolitan district consists of more than one county, if a majority of the votes cast on the proposal by the qualified voters voting in a county proposed for inclusion in the metropolitan district are in favor of the proposal, then the metropolitan district shall be deemed organized and that county shall be included in the metropolitan district, but if a majority of the votes cast on the proposal by the qualified voters voting in the county proposed for inclusion are opposed to the proposal, then the county shall not be included in the metropolitan district. After the metropolitan district has been created, counties eligible for inclusion in the metropolitan district and not already included in the metropolitan district may join the metropolitan district after such a proposal is submitted to the voters of the county proposed for subsequent inclusion and such proposal is approved by a majority of the qualified voters voting thereon in the county proposed for inclusion in the manner described in this section and subsection 1 of section 67.1715 and in section 67.1718.
§ 67.1721, RSMo