Resident of Norway wanting to sell spreadshirts to the US


I am setting up the shop with a .com domain to be able to sell designs in the US. I am therefore filling out the W-8BEN. Under “Foreign Tax identifying number” I am wondering what to fill out. It says “See instructions”, but I can not find any. Same with Part II.

I would like to fill this form out properly the first time around. Also, is there any danger of a “surprise” tax coming my way after filling this out and starting to sell? Where can I read information about selling specifically as a Norwegian resident trough Spreadshirt in the US. Appreciate any help I can get. Perhaps there are any Norwegians on here who has done this already?