Click on the Forums tab to bring up the forums overview page.
Click on Post New Thread near the top
Select the forum to post to
Scroll down in the CreateThread page to the Post a poll section.
Since the thread is not "yours" a poll might be viewed as a tangent. You could start a new thread with the poll on that tangent and link to the original thread. I think allowing a respondent to add a poll to an existing thread is a major modification of the TS's intent and could open the door to unintended consequences.