In the web interface go to account settings and then click on the 
next to your name in the “Profile(s)” section. At the bottom of the popup you should see a link “Combined CRS/IRS Tax Form”. Clicking on that you get the “Substitute Form W-8BEN”. If you see that and it’s filled out, you’re all set.
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Thanks for the tip, much appreciated! It says “No” for me having a US taxpayer ID number (Question 6) and “Yes” for me qualifying for the benefits of a US income tax treaty (Question 7), rest is just basic information about me, so I guess these answers mean I am good or did I fill it wrong (esp (7)). Cheers