I want this too! We're using Flarum for our forum, it's an open source forum technology. It's awesome, but it has some flaws, the development and growth is a bit slow, so new features and integrations are being added very slowly. But they do have plans to develop Polls in a form of extension. They just don't have an ETA for it yet, sorry!

A nice workaround would be creating a 7-day poll on Twitter and then copying a link to that tweet into new forum thread and asking people to vote or share their opinions in the comments.