Mo. Rev. Stat. § 491.250: Application for writ.

An application for such writ shall be verified by affidavit, and shall state the title and nature of the proceeding in which the testimony of the prisoner is desired, the court or officer before whom pending, and that the testimony of such prisoner is material and necessary to the applicant on the trial or hearing of such suit or proceeding, as he is advised by counsel, and verily believes.