Hi nikitn,
On the first page it asks you to show the amount of Foreign Tax Credit Relief you are looking to claim.
Enter the figure there and put a cover letter along with the evidence for us to see where the tax has come from.
Thank you.
Amended response
No, if you are a UK resident you do not have to declare or tell us about any income or capital gains from ISA Savings and investments, including dividend income, you can see the guidance here:
SAIM2310 - Interest: exemptions: tax-free savings income: ISAs, PEPs and CTFs
You will need to approach the Canadian government direct regarding any claim against the withholding tax they have deducted on your ISA dividends. They may request you to obtain a certificate of residence from us verifying you are a resident in the UK for tax purposes. If they do, you can apply for a certificate of residence here:
How to apply for a certificate of residence to claim tax relief abroad
Thank you.