These additional financial sponsors also have to submit an Affidavit of Support, proof of their income, and proof of their legal status in the United States. If you want to sponsor someone for a family-based green card, you need to provide proof of a U.S. domicile in your Affidavit of Support (Form I-864).Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support. Completing and signing Form I-864 makes you the sponsor. This guide will explain how to prove domicile when filling out Form I-864 ("Affidavit of Support"). Country of Domicile enter the joint sponsor's country of domicile (most sponsors will enter "US" as the country of their domicile). 6. Proof of US citizenship or permanent resident status. Federal tax return or IRS tax transcript for the most recent year.