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).If the petitioner does not have a domicile in the United States, can a joint sponsor file an I864? 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. Country of Domicile enter the joint sponsor's country of domicile (most sponsors will enter "US" as the country of their domicile). 6. If you live outside the US but claim to be a US citizen, you will need to send in a written explanation and proof of your claim along with your Form I864.