Mass. Gen. Laws ch. 167D, § 4
(b) Notwithstanding subsection (a), a holder of the account may provide for limited access to the account by another person to act as a signatory to the account pursuant to a declaration of intent in the form of a written statement, signed and sworn to by the account holder, evidencing the account holder's intent to designate another person as signatory to the account for the purpose of exercising, on behalf of the account holder, such powers with respect to the account as shall be expressed in the declaration.
The declaration of intent shall include:
(c) The provisions of the declaration relative to the account shall become effective upon the filing of the declaration with the financial institution, if the following documents are executed contemporaneously with, or on the same document as the declaration:
(3) a statement, signed and sworn to by the signatory, acknowledging receipt of an attested copy of the declaration of intent and the statement required by clause (2).
The declaration submitted to effect the establishment of the account, and documents related thereto, shall be maintained by the financial institution with the records of the account.
(f)