Mo. Rev. Stat. § 507.170
Current with changes from the 2023 Legislative Session
The petition for the appointment of a next friend, the written consent of the person proposed to be next friend, and the order of appointment, shall be filed in the office of the clerk of the court where the suit is proposed to be brought, before any proceedings shall be had in the cause.
§ 507.170, RSMo
RSMo 1939 § 865