Upvote:2
This is answered in section 3 of the UKVI guidance:
Your biometric confirmation receipt will be stamped at your appointment. You then have 5 days to post this, your application form, your passport and supporting documents to UK Visas and Immigration (UKVI). All applications must be accompanied by a valid passport with at least one page without any endors*m*nts. If the application arrives at the decision making centre without a passport the application will be refused.
You should post your application to:
Non-settlement applications
UK Visas and Immigration
British Consulate General
845 Third Avenue
New York
10022...
3.1 Return shipping
To have your documents returned you must provide a fully addressed prepaid electronic shipping label (we cannot accept ground services and non-electronic waybills). Put your return shipping label and envelope inside the package, and keep a copy of the waybill for your records.
See the answer to this question on how the applications should be mailed.