Mo. Rev. Stat. § 295.110
Whenever, after the effective date of this chapter, a situation exists in any utility whereby employees are rendering services under terms and conditions which were not at the time this chapter becomes effective and which have not heretofore been the subject of the contract, and said employees desire to effectuate a change in the terms of employment or a utility desires to effectuate a change in said terms of employment then and in that event, it shall be the duty of the party desiring such change, not less than sixty days prior to the desired effective date thereof, to inform the other party in writing of the specific changes so desired in the manner in which they are desired, either by written contract or otherwise and to file a copy of such terms with the state board of mediation.
§ 295.110, RSMo