Treas. Reg. §301.6109-1(c) would apply here.
(c) Requirement to furnish another's number.
Every person required under this title to make a return, statement, or other document must furnish such taxpayer identifying numbers of other U.S. persons and foreign persons.........If the person making the return, statement, or other document does not know the taxpayer identifying number of the other person, such person must request the other person's number. The request should state that the identifying number is required to be furnished under authority of law. When the person making the return, statement, or other document does not know the number of the other person, and has complied with the request provision of this paragraph (c), such person must sign an affidavit on the transmittal document forwarding such returns, statements, or other documents to the Internal Revenue Service, so stating..........
