Click on "Advanced..." (I forget what the full name is) after you open up the window that lets you start a new thread, and then scroll to the bottom. That's where that choice is.
And warning. It gives no room to explain your poll, so it's a bit wonky. Feel free to explain as the first response to the poll.
(Helpful hint: Or just do it in a regular post, and get people to answer that way. Much easier, and people will either let their opinion be known with their names or they won't participate. Personally, I'd rather not have people participate if they think they need to hide who they are when answering. But that's just a helpful hint with an editorial added.)
(And, as the world's slowest typist, now I find out how many answered simply before I was ready to click "Post Quick Reply." Why does anyone thing me typing is ever quick? lol)