Do I need a Taxpayer ID Number (TIN) to avoid tax if I'm from Canada/NZ?

Do I need a Taxpayer ID Number (TIN) to avoid US double tax if I’m from Canada (currently in NZ)? Both countries have tax treaties with the US but I’m not sure whether I’ll need to apply for a TIN or not. I will be trading with forex.com which is American based.

Thanks in advance!