How do I get my tax code if I do not currently work, earn or need to pay tax?

I'm currently setting up an Adobe Stock Contributor account in which I have been asked to fill in a W-8 BEN form which requires my foreign tax identification number (which I presume is my UK tax code). I'm unsure how to get this as I have not started to earn yet and therefore have no need to do a self-assessment form, I also am not setting up a business as I don't think I need to? And have only got tax codes from previous employment which I believe expire (this was 4 years ago).

Hi

Your foreign tax identification number can be either your Self Assessment Unique Taxpayer Reference (UTR), or your National Insurance Number (NINO).
You should be able to view your NINO on any letter issued to you by HMRC.

Thank you