Mo. Rev. Stat. § 486.110

Every such commissioner, before performing any duty or exercising any power in virtue of his appointment, shall take and subscribe an oath or affirmation before some judge or clerk of any United States court of record or before some judge or clerk of any court of record in and of the state of Missouri, or before a judge or clerk of one of the courts of record of the district, territory, state or county in which said commissioner shall reside, well and faithfully to execute and perform all the duties of such commissioner, under and by virtue of the laws of the state of Missouri; which oath, and a description of his seal of office, if there be one, together with his signature thereto, shall be filed in the office of the secretary of state of this state within six months after the date of his appointment.