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).One of the requirements for Form I864 is showing that you are domiciled in the US. In other words, your principal residence must be located in the US. So in the box asking for the document "proof of domicile" I put his most recent tax return (not the transcript). This proof is essential when you submit Form I864, the Affidavit of Support, as part of the green card application process. On Form I864, petitioners must prove that they have the ability to financially support the visa applicant(s) if necessary.