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).Line-by-line guidance for properly filling out USCIS's I-864 Affidavit of Support to sponsor an immigrating family member. Each joint sponsor must fill out a separate I864 form. In their form, they will include the names of all immigrants that they will personally be sponsoring. So in the box asking for the document "proof of domicile" I put his most recent tax return (not the transcript). Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support.