The checkbox is a recent addition to the 4506 Tax Form. It appeared without warning. And the lender's head will explode if you miss having it checked (if lender's had heads). It's right above the signature line:
Signature of taxpayer(s).
I declare that I am either the taxpayer whose name is shown on line 1a or 2a, or a person authorized to obtain the tax return requested. If the request applies to a joint return, at least one spouse must sign. If signed by a corporate officer, 1 percent or more shareholder, partner, managing member, guardian, tax matters partner, executor, receiver, administrator, trustee, or party other than the taxpayer, I certify that I have the authority to execute Form 4506 on behalf of the taxpayer. Note: For tax returns being sent to a third party, this form must be received within 120 days of the signature date.
--> X <-- Signatory attests that he/she has read the attestation clause and upon so reading declares that he/she has the authority to sign the Form 4506.
Sign Here > _________________________________________________________________