Mo. Rev. Stat. § 21.130
The election authority to whom any writ of election is delivered shall cause the election to supply the vacancy to be held within the limits composing the county or district at the time of the next preceding general election, and shall issue its proclamation or notice for holding the election accordingly, and transmit a copy thereof, together with a copy of the writ, to* the election authority of each of the counties within which any part of the old county or district lies, who shall cause copies of the notice to be put up, and the election to be held accordingly, in the parts of their respective counties as composed a part of the old county or district for which the election is to be held, at the last preceding general election; and the returns shall be made and the certificate of election granted in all things as if no division had taken place.
§ 21.130, RSMo