755 ILCS 5/11a-6
A person, while of sound mind and memory, may designate in writing a person, corporation or public agency qualified to act under Section 11a-5, to be appointed as guardian or as successor guardian of his person or of his estate or both, in the event he is adjudged to be a person with a disability . The designation may be proved by any competent evidence, but if it is executed and attested in the same manner as a will, it shall have prima facie validity. If the court finds that the appointment of the one designated will serve the best interests and welfare of the ward, it shall make the appointment in accordance with the designation. The selection of the guardian shall be in the discretion of the court whether or not a designation is made.
755 ILCS 5/11a-6