Can someone tell me what I should put on top of the support letter?

My fiance ask me what address, name he should put on top of the support letter.
Since he is getting it all type up in UK, I don't have a printer and I have not yet apply for visa yet.

I don't know what is the address should be on top.
To who?  Custom Clearance Officer?

I am planning to send the spouse visa application in NYC.

Also if anyone know how long the notary passport is valid for after my fiance gets one?  Can he get it now but I won't be applying for visa till months later.

Customs deals with the importation of goods & materials, not visas.

Address it to the Entry Clearance Officer, a formal address location is not needed.

oops...I type too fast and wasn't thinking before I type custom.. 
thanks for replying!