You need the
<iframe> code instead.
Go to your poll at polleverywhere then click on "3. Present" > Share > Embed poll. There will be a place that says "Copy and paste this snippet into your web page or blog". Copy a code that would look like:
<iframe src="https://embed.polleverywhere.com/multiple_choice_polls/XXX" width="100%" height="100%" frameBorder="0"></iframe>
and enter this to your slide. Just a note that open your
html output in a browser like Chrome and not the Viewer pane in RStudio because the latter doesn't show it.
Alternatively if you want people to answer the question without leaving the presentation slide you can embed it as
<iframe src="https://PollEv.com/multiple_choice_polls/XXX/web" width="100%" height="100%" frameBorder="0"></iframe>
replacing src url from "Shareable response link". It works from Chrome but Firefox seems to squish a bit. Users have to identify who they are first though.