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).This guide will explain how to prove domicile when filling out Form I-864 ("Affidavit of Support"). Consular processing support without the high price tag. Completing and signing Form I-864 makes you the sponsor. So in the box asking for the document "proof of domicile" I put his most recent tax return (not the transcript). Line-by-line guidance for properly filling out USCIS's I-864 Affidavit of Support to sponsor an immigrating family member. This form requires the petitioner to show they have a domicile in the United States and enough money or assets to assist the immigrant.