Wis. Stat. § 52.18: Authorization and witnesses.

(1) An adult with a functional impairment and his or her supporter entering into a supported decision-making agreement must sign and date the agreement in the presence of 2 or more subscribing witnesses or a notary public.

(2) If the adult with a functional impairment and his or her supporter choose to sign and date the supported decision-making agreement before witnesses, the attesting witnesses must be at least 18 years of age.