Mont. Code Ann. § 85-8-803

In case of failure to serve any notice of any proceeding or hearing provided for in this chapter upon any person or corporation, such person or corporation may appear in open court and waive such defect of service or may waive it by filing in court or delivering to the commissioners of the drainage district, to be filed in court, a written waiver of such defect, in which waiver said defect shall be described, which waiver shall be signed by such party and witnessed and acknowledged before a proper officer having power to take acknowledgments of deeds.