Tableau Online has security in place such that only authenticated users may log onto it. Tableau Server has guest access (if you have the right licensing) which means you can make views available publicly. However, to my knowledge, this is not available in Tableau Online. Thus, when you embed, it will require people to authenticate first. If your goal is to embed vizzes such that they are available to the public then I'd strongly suggest using Tableau Public. Tableau Public is a free to use, publicly available instance of Tableau Server. I use it for all my personal projects and it embeds into my website quite easily.