Mo. Rev. Stat. § 521.770: Third-party claim to be in writing.

No claim made to any personal property levied on as aforesaid shall be valid or lawful as against such officer, unless such claimant or his agent shall set forth his claim in writing, verified by the affidavit of such claimant or his agent, describing the property claimed and stating his interest therein and whether it is in the whole or only part thereof and stating also that he is in good faith the lawful owner of the interest claimed by him in said property; that the defendant in such execution or attachment has no right or title, directly or indirectly, in the interest in said property claimed by said claimant; and that such claim is not made in collusion with said defendant for the purpose of vexing, hindering or delaying the plaintiff in obtaining his just rights.