Mont. Code Ann. § 53-21-1305

If the directive appoints an agent, the agent may accept the appointment by signing the directive. The agent's signature does not have to be witnessed, notarized, or otherwise validated. The lack of agent's signature does not affect the validity of the directive. The directive may provide for one or more alternate agents. The authority of the agent is in effect only after a determination of incapacity as provided in this part and only for as long as the period of incapacity.