when a supporting statement is required either it will either be described in the code section with what it must contain and other details as needed or make reference to the regulations that will describe what must be done.
section 721(c) only comes into play if upon disposition of the contributed property gain will be allocated to a non-US person.
if 721(c) does into play the reg 1.721(c)-6 describes what is required from the parties involved.
it certainly is not wrong to include a statement with your return
