Mo. Rev. Stat. § 506.110: How suits may be instituted in courts of record.

1. Suits may be instituted in courts of record, except when the statute law of this state otherwise provides, either:

(1) By filing in the office of the clerk of the court a petition setting forth the plaintiff's cause or causes of action, and the remedy sought, and by the voluntary appearance of the adverse party thereto; or

(2) By filing such petition in such office, and suing out thereon a writ of summons against the person or of attachment against the property of the defendant.

2. The filing of a petition in a court of record, or a statement or account before a court not of record, and suing out of process therein, shall be taken and deemed the commencement of a suit.