Form 8233 is not submitted to the IRS by the taxpayer at all. A nonresident alien claiming a tax-treaty exemption on personal services income gives the form to the withholding agent — the employer or payer — who reviews it, signs Part IV and forwards a copy to the IRS within 5 days per the instructions.
If you're the withholding agent handling the forwarding, follow the address in the current Form 8233 instructions.
Give it to the withholding agent before the payment; a new form is needed each tax year.