I have a question regarding the selling process on Amazon

Hey! I recently started up a seller account on Amazon, and need help setting up the product I have chosen out. I have some things I wanna sell, and am to the part where it asks for External Product ID.

I understand that I am to go to GS1.org and purchase keys. Is this correct? I am going to ship in products from Alibaba, directly to the warehouse, so FBA. How do I do the process of purchasing the keys, and using them. Do I use ASIN or the EAN/.. option?

Thanks for the help guys.