Hi,
You will need to request a certificate of residence at
How to apply for a certificate of residence to claim tax relief abroad, declaring the dividends and amount received in pounds sterling.
You cannot include a future date.
You can submit this to the IRS along with their repayment claim forms, to claim back any overpaid tax.
You will need to declare the foreign dividends and tax paid in SA106 and claim up to 15% foreign tax credit relief.
Thank you.