You'll need to provide your national insurance number for Dodl’s regulatory and reporting requirements, set by the Financial Conduct Authority (FCA) and HMRC. It’s also how Dodl ensures you get all the lovely tax benefits for your ISA(s) and/or pension.
You can usually find your national insurance number on your payslip, P60, or letters from your pension provider and HMRC. If you still can't find it, the government has a tool that can help.