Mo. Rev. Stat. § 452.782
Current with changes from the 2023 Legislative Session
If the court learns from information furnished by the parties under section 452.800 or from other sources that a person not a party to the custody proceeding has physical custody of the child or claims to have custody or visitation rights with respect to the child, it may order that person to be joined as a party and to be duly notified of the pendency of the proceeding and of his or her joinder as a party. If the person joined as a party is outside this state, such person shall be served with process or otherwise notified in accordance with section 452.762.
§ 452.782, RSMo